fix(agent): map native google_search and surface empty rounds

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.

src/agent_loop.py

@@ -2161,6 +2161,13 @@ async def stream_agent_loop(
         # Separator in accumulated response
         full_response += "\n\n"
 
+    # If the response is completely empty and no tools were executed,
+    # yield a fallback message so the user is not left hanging.
+    if not full_response.strip() and not tool_events:
+        _error_msg = "The model returned an empty response. Please try again or switch to a different model."
+        yield f'data: {json.dumps({"delta": _error_msg})}\n\n'
+        full_response = _error_msg
+
     # --- Final metrics ---
     total_duration = time.time() - total_start
     metrics = _compute_final_metrics(