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_schemas.py

@@ -1074,6 +1074,7 @@ def function_call_to_tool_block(name: str, arguments: str) -> Optional[ToolBlock
         return None
 
     tool_type = _TOOL_NAME_MAP.get(name, name)
+
     # Allow MCP tools through (namespaced as mcp__serverid__toolname)
     if tool_type.startswith("mcp__"):
         content = json.dumps(args) if args else "{}"
@@ -1093,7 +1094,13 @@ def function_call_to_tool_block(name: str, arguments: str) -> Optional[ToolBlock
     elif tool_type == "python":
         content = args.get("code", "")
     elif tool_type == "web_search":
-        content = args.get("query", "")
+        queries = args.get("queries")
+        if isinstance(queries, list) and queries:
+            content = str(queries[0])
+        elif queries:
+            content = str(queries)
+        else:
+            content = args.get("query", "")
     elif tool_type == "read_file":
         content = args.get("path", "")
     elif tool_type == "write_file":