Refactor dotnet-watch into a package consumable from Aspire.CLI

[tmat]

Requires dotnet/roslyn#80556, #51143

Splits dotnet-watch into two components: dotnet-watch CLI tool itself and Microsoft.DotNet.HotReload.Watch package with the core implementation.
The package does not include CLI parsing (no dependency on dotnet CLI commands) and legacy "no hot reload" mode.

The change also includes Microsoft.DotNet.HotReload.Watch.Aspire project that has IVT to Aspire.CLI (similar to Roslyn's External Access) and allows Aspire to launch the watcher.

[github-actions]

This PR is targeting main, which is now for .NET 11-facing work. If you intended to target .NET 10, either retarget this PR to release/10.0.1xx or make sure you backport the change to release/10.0.1xx after merging. See #50394 for more details.

[dotnet-policy-service]

Thanks for your PR, @@tmat.
To learn about the PR process and branching schedule of this repo, please take a look at the SDK PR Guide.

[Copilot]

Pull Request Overview

This PR refactors the dotnet-watch tool into a consumable package for Aspire.CLI by splitting it into two main components: the original CLI tool and a new Microsoft.DotNet.HotReload.Watch package containing the core implementation.

• Extracts the core watch functionality into a reusable library package
• Creates a new Aspire-specific project with IVT access for external consumption
• Removes CLI-specific dependencies from the core package to eliminate command parsing logic

Reviewed Changes

Copilot reviewed 52 out of 98 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/BuiltInTools/Watch/Microsoft.DotNet.HotReload.Watch.csproj	New core watch library project containing shared functionality
src/BuiltInTools/Watch.Aspire/Microsoft.DotNet.HotReload.Watch.Aspire.csproj	New Aspire-specific executable package for external consumption
src/BuiltInTools/dotnet-watch/dotnet-watch.csproj	Updated to reference the new core library instead of duplicating code
test/Microsoft.DotNet.HotReload.Test.Utilities/	New shared test utilities project extracted from existing tests
Various test files	Updated imports and references to use the new shared test utilities

[DustinCampbell]

I dug through the whole change, and it looks good.

[tmat]

Moving to 10.0.2xx branch: #51123