Watch code generation overlays for hot regeneration

[Copilot]

Summary

When an applied overlay file changes, generation now reruns the same way it does for OpenAPI source edits. This closes the gap where overlay-driven changes required manual regeneration.

• Watch scope expanded

  • CodeGenerator.watch() now watches all local generation inputs, not just the OpenAPI file:
    • local OpenAPI file + overlays for file-based specs
    • overlays only for HTTP OpenAPI sources
  • watch remains a no-op when there are no local watchable paths.

• Overlay change regression coverage

  • Added a focused server test that edits an applied overlay during watch mode and asserts regenerated type output changes accordingly.

const watchablePaths = this.openapiPath.startsWith("http")
  ? [...this.overlays]
  : [this.openapiPath, ...this.overlays];

Original Prompt

<issue_title>Watch changes to overlay files</issue_title>
<issue_description>When an applied overlay file changes, the code should regenerate, just like when the OpenAPI document itself changes.</issue_description>

Manual acceptance tests

• Run generator watch mode with a local OpenAPI file and one overlay, edit only the overlay, and observe regenerated output updates automatically.
• Run generator watch mode with an HTTP OpenAPI source plus a local overlay, edit only the overlay, and observe regeneration.
• Run generator watch mode with an HTTP OpenAPI source and no overlays, and observe that no local file watch behavior is started.
• Edit only the OpenAPI source (no overlay edits) and confirm existing regeneration behavior is unchanged.

Tasks

• Update CodeGenerator.watch() to watch overlay files in addition to the OpenAPI source when applicable.
• Preserve existing no-op watch behavior when no local files are watchable.
• Add targeted test coverage for regeneration triggered by overlay file edits.