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/tool_execution.py

@@ -788,7 +788,7 @@ async def execute_tool_block(
             result = {"error": "MCP manager not available", "exit_code": 1}
     else:
         desc = f"unknown: {tool}"
-        result = {"error": f"Unknown tool type: {tool}"}
+        result = {"error": f"Unknown tool type: {tool}", "exit_code": 1}
 
     logger.info(f"Tool executed: {desc} -> exit_code={result.get('exit_code', 'n/a')}")
     return desc, result