🧪 [testing improvement] Add unit tests for MetalGuestGpuBackend.TryCompileVertexShader

[manupawickramasinghe]

🎯 What: The testing gap addressed
Tests were missing for MetalGuestGpuBackend.TryCompileVertexShader, which handles compiling vertex shaders into MSL for the Metal GPU backend. The complexity of shader state creation and translation made this a critical path lacking coverage.

📊 Coverage: What scenarios are now tested

• Added TryCompileVertexShader_EmptyProgram_Fails to verify that attempting to compile an empty or invalid shader program gracefully fails and returns an appropriate error message.
• Added TryCompileVertexShader_WithValidProgram_Succeeds to verify that a well-formed vertex shader program successfully translates to a MetalCompiledGuestShader instance without errors.

✨ Result: The improvement in test coverage
MetalGuestGpuBackend now has direct tests for its primary compilation entrypoint, improving confidence in the MSL translation layer and guarding against future regressions in state handling.

---

PR created automatically by Jules for task 9871708352247430781 started by @manupawickramasinghe

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.