fix(cli): better runtime handling for ui:start

[cabljac]

This should resolve #3338

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[Copilot]

Pull Request Overview

This PR improves runtime handling for the ui:start command by implementing better detection and configuration for different JavaScript runtimes (Node.js, Bun, and compiled binaries). The goal is to resolve compatibility issues when spawning the UI server process across different execution environments.

• Adds comprehensive runtime detection for Node.js, Bun, and compiled binaries
• Implements proper spawn configuration with platform-specific handling
• Improves error handling and validation for runtime execution paths

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
runtime-detector.ts	Implements runtime detection logic with support for Node.js, Bun, and compiled binary detection
spawn-config.ts	Provides spawn configuration building and executable path validation
ui-start.ts	Updates the UI start command to use the new runtime detection and spawn configuration
runtime-detector_test.ts	Comprehensive test coverage for runtime detection functionality
spawn-config_test.ts	Test coverage for spawn configuration building and validation

[cabljac]

Seems like it doesn't work right now on Windows at least, investigating further.

[MichaelDoyle]

@pavelgj is OOO right now, so I am going to tap in @apascal07 to help with the review if possible.

One thing that jumps out to me, without going too deep, we use the term "runtime" in two different contexts now. The "Genkit" runtime and the "Javascript" runtime.

So we should at least make sure the error messages refer to the Javascript runtime so that its clear to us/to the user what this is. Potentially we could consider naming the utility classes/methods accordingly as well.

[apascal07]

Seems like it doesn't work right now on Windows at least, investigating further.

Will review once it's all there, thanks!

[cabljac]

The windows issues are fixed, I'll rename the classes and logs etc, should be ready for review end of day tomorrow