Summary

The Gemini API enforces strict validation on function_declarations and rejects requests containing unsupported JSON Schema keywords such as additionalProperties, propertyNames. This caused Write mode to fail with "failed to stream completion" when tools with complex schemas were used.

This PR strips these unsupported keywords from tool schemas before sending them to the Gemini API in adapt_to_json_schema_subset.

How to Review

• Check crates/language_model/src/tool_schema.rs — the adapt_to_json_schema_subset function now removes additionalProperties and propertyNames from schemas.
• Tests are added covering removal of these keys and nested schema handling.
• To reproduce the original issue, send a tool schema containing propertyNames or additionalProperties to the Gemini API — it returns HTTP 400 INVALID_ARGUMENT

How to Test

Run the unit tests:

cargo test -p language_model

OR manually reproduce this using ->

curl -s "https://generativelanguage.googleapis.com/v1beta/models/gemini-1.5-flash:generateContent?key=YOUR_KEY" \
  -H 'Content-Type: application/json' \
  -d '{"contents":[{"parts":[{"text":"test"}]}],"tools":[{"functionDeclarations":[{"name":"test","parameters":{"type":"OBJECT","properties":{"field":{"type":"OBJECT","propertyNames":{"pattern":"^[a-z]+$"},"additionalProperties":{"type":"STRING"}}}}}]}]}'

Closes #52430

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Video

Note : Reopens previous work from closed PR #52644 (fork was deleted)

Release Notes:

• Fixed an issue where Gemini models would not work when using specific MCP servers