feat(ide-mode): add diagnostic info to IDE context

[Mond45]

Summary

Added diagnostic information from the active VSCode file to the IDE context while gemini-cli is running in IDE mode.

Details

Gemini Client:

• To conserve context, include diagnostic information (start/end line/column, message, severity) only for the active file in the editor.
• Include full diagnostic information on the first query and whenever the active file changes.
• On subsequent queries, send only the diagnostic delta (diagnostics added or removed for the active file).

VSCode Companion Extension:

• Watch for diagnostic changes on tracked open files (up to a maximum of 10).

Related Issues

Closes #6688

How to Validate

• Run gemini-cli with debug mode on.
• Ask something related to errors in the file, e.g., “Fix the syntax errors in the currently opened file.”
• Observe the added diagnostic info in the IDE context in the debug log.
• Make changes to the file that add or remove diagnostic items.
• Submit new queries and observe the diagnostic info delta.

Pre-Merge Checklist

• Updated relevant documentation and README (if needed)
• Added/updated tests (if needed)
• Noted breaking changes (if any)
• Validated on required platforms/methods:
  • MacOS
    • npm run
    • npx
    • Docker
    • Podman
    • Seatbelt
  • Windows
    • npm run
    • npx
    • Docker
  • Linux
    • npm run
    • npx
    • Docker