### 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:
```sh
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

- [x] 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](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Video 

[Screencast from 2026-03-29
08-32-18.webm](https://github.com/user-attachments/assets/a0069f0e-1f2b-45dc-85bf-f24aacb08599)


### 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