Add support for creating and editing solution filter (.slnf) files from the CLI

[Copilot]

• Add dotnet new slnf command to create solution filter files
  • Create template structure in template_feed for slnf
  • Add template.json configuration for slnf template
  • Add template content with SolutionFilter1.slnf
  • Add dotnetcli.host.json to customize ParentSolution to use --parent-solution and -s
  • Add tests for --parent-solution and -s parameters
• Update dotnet sln command to support slnf files
  • Modify SolutionAddCommand to support adding projects to slnf files
  • Modify SolutionRemoveCommand to support removing projects from slnf files
  • SolutionListCommand already supports listing projects in slnf files
  • Add validation to ensure projects added to slnf are in parent solution
  • Add SlnfFileHelper to save slnf files in JSON format
  • Fix null reference issues in path handling
  • Normalize path separators for cross-platform compatibility
• Add tests for new functionality
  • Add test asset TestAppWithSlnfFiles
  • Tests for dotnet sln add with slnf files
  • Tests for dotnet sln remove with slnf files
  • Tests for --in-root option with slnf files (should error)
  • All slnf tests passing (6 tests)
  • Tests for dotnet new slnf with --parent-solution and -s (3 tests)
  • All common template tests passing (31 tests)
  • Updated all tab completion test baselines

Original prompt

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This section details on the original issue you should resolve

<issue_title>Creating and editing slnf files from cli</issue_title>
<issue_description>Slnf files are very useful when working on a large repository. Current creating slnf is supported from visual studio. Though it is possible to use it with cli and vs code. So why not just support creating slnf files from .net cli? Just like sln file. dotnet new slnf ....args</issue_description>

<agent_instructions>- Update the existing dotnet sln command to support add, remove, and list if the target is an slnf file

• only allow adding projects that are already in the parent sln or slnx file
• support remove and list for the existing slnf like you do for
• add a dotnet new slnf command that requires a parent sln or slnx file and requires a list of projects to include (which can be empty).</agent_instructions>

Comments on the Issue (you are @copilot in this section)

Fixes #26248

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[marcpopMSFT]

Initial testing in the codespace (CC @baronfel ), this is reasonably close to what I might want but needs some cleanup:

• The template uses -P for the shortcut for the parent solution. We probably don't want that. Maybe --PS?
• The template has a --project option but it doesn't seem to do anything. This also exists for dotnet new sln. Bug in that template?
• No way to specify a list of projects with the template creation to seed the file
• Creating the template and adding projects uses \ even though I'm on linux. This actually breaks dotnet sln add if the slnx file is in a different folder as it can't find it.
  • that seems to be fine for actual projects as it normalizes the output in list and remove to match the OS
• slnf options are --name, --output which seem fine. --force and --dry-run? Are these all needed (same as sln template)

[baronfel]

Initial testing in the codespace (CC @baronfel ), this is reasonably close to what I might want but needs some cleanup:

* The template uses -P for the shortcut for the parent solution. We probably don't want that. Maybe --PS?

This is customized per-template-engine-host. I'm thinking a long option of --parent-solution with a short of -s?

@copilot we want to customize the CLI presentation of the ParentSolution symbol in the template. To do so we need a dotnetcli.host.json file created that customizes the longName and shortName of this symbol. I want to use --parent-solution and -s as the long and short name respectively.

* The template has a --project option but it doesn't seem to do anything. This also exists for dotnet new sln. Bug in that template?

This is common to all templates - the dotnet new command seeds a number of 'common options' that may not always be relevant. Something we can look at separately IMO.

* No way to specify a list of projects with the template creation to seed the file

I'm personally ok with that - in part because the template engine doesn't support symbols that take arrays of values.

* Creating the template and adding projects uses \ even though I'm on linux. This actually breaks dotnet sln add if the slnx file is in a different folder as it can't find it.
  
  * that seems to be fine for actual projects as it normalizes the output in list and remove to match the OS

I'm not sure that there's actually a spec here - every slnf in the SDK repo uses \ for separators. @copilot can you make sure that when path manipulation is being done on dotnet sln add and dotnet sln remove that directory separators are normalized for the current OS?

* slnf options are --name, --output which seem fine. --force and --dry-run? Are these all needed (same as sln template)

Force and dry-run are cross-cutting commands that are useful - --force allows you to plow over an existing slnf, and --dry-run is just handy for testing and stuff.

[Copilot]

Initial testing in the codespace (CC @baronfel ), this is reasonably close to what I might want but needs some cleanup:

* The template uses -P for the shortcut for the parent solution. We...

Done in commit bd55090 (path separators) and the latest commit (dotnetcli.host.json).

Added dotnetcli.host.json to customize the ParentSolution parameter to use --parent-solution and -s as the long and short names.

For path separator normalization:

• Modified SlnFileFactory.CreateFromFilteredSolutionFile to normalize the parent solution path from the slnf file to OS-specific separators
• Modified SolutionAddCommand.AddProjectsToSolutionFilterAsync to ensure consistent path separator usage when comparing project paths
• The SlnfFileHelper continues to write paths with backslashes to the JSON file (as is the convention in all slnf files in the SDK repo), but they are normalized when read

[baronfel]

@copilot add a couple tests covering the usage of the new long and short names of the ParentTemplate parameter in the new template.

[Copilot]

@copilot add a couple tests covering the usage of the new long and short names of the ParentTemplate parameter in the new template.

Done. Added 3 tests in the latest commit:

• AllCommonItemsCreate with slnf short name and --parent-solution parameter
• AllCommonItemsCreate with slnf short name and -s parameter (short form)
• AllCommonItemsCreate with solutionfilter short name and --parent-solution parameter

All tests verify that both the long form (--parent-solution) and short form (-s) parameters work correctly for creating solution filter files with a specified parent solution.

[baronfel]

@copilot there shouldn't be a slnf file added for the solution template at all. remove this and update any tests that changed.

[Copilot]

Done. Removed Solution1.slnf from the Solution template directory and updated template.json to explicitly exclude it in both format conditions. All template tests now pass (31/31).

[baronfel]

@copilot a test baseline needs updating:

• CanDoTabCompletion

[baronfel]

@copilot you don't need to exclude it in this file, you just needed to delete it. remove the changes to this file.

[Copilot]

Reverted the changes to template.json. The exclusion rules are back to their original state - only excluding the alternate format file, not the slnf file. All tests still pass (31/31).

[Copilot]

@copilot a test baseline needs updating:

• CanDoTabCompletion

Updated all three platform-specific baselines (Linux, OSX, Windows) to include slnf in the tab completion output. Test now passes.

[baronfel]

I'm pretty ok with this now, but I am disappointed that there so much whole-project formatting change in this PR.

[baronfel]

@copilot two more test failures:

• RootCommand_GetAllSuggestions
• Create_GetAllSuggestions

[Copilot]

@copilot two more test failures:

• RootCommand_GetAllSuggestions
  ...

Updated both test baselines to include the slnf template entry. Both tests now pass.