Models (notably Gemini) emit a native 'google_search' function call, but the
agent loop had no mapping for it, so the call failed to convert, the round
produced 0 chars and 0 tool blocks, and generation died silently — the web
client hung on 'waiting for first token' with no error (also #443).
- Map google_search / google_search_retrieval / google_search_grounding to the
web_search tool, and read Gemini's 'queries' array (falling back to 'query').
- In stream_agent_loop, when a round yields no response text and no tool
events, emit a visible fallback message instead of leaving the user hanging.
- Give the unknown-tool execution branch an explicit exit_code=1 so the failure
is logged as an error rather than 'n/a'.
Unknown/unconvertible tool names still return None (unchanged) so they are
dropped safely rather than executed. Added tests covering the google_search
mapping, the queries array, and unknown/invalid-JSON returning None.
