feat: add web search gateway tool

[franciscojavierarceo]

Summary

• Adds gateway-managed web_search_preview support backed by You.com via YOU_API_KEY, normalizing the tool to the web_search function shape expected by the model backend.
• Executes web search calls server-side, feeds function_call_output back into follow-up model rounds, and returns OpenAI-compatible web_search_call output items to clients.
• Emits web search lifecycle SSE events during streaming tool loops, executes multiple gateway tool calls concurrently, and aligns the gateway tool loop limit to 10 rounds.
• Caveat: this PR uses mocked LLM and You.com integration coverage. A follow-up PR will add recorded cassettes once we have a live gpt-oss/vLLM environment available for end-to-end recordings.

Test Plan

• cargo test
• cargo clippy --all-targets -- -D warnings
• cargo fmt -- --check
• git diff --check
• Not run: live You.com API smoke test because YOU_API_KEY was unset in the local environment.