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(

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

src/tool_parsing.py

@@ -95,6 +95,9 @@ _TOOL_NAME_MAP = {
     "search": "web_search",
     "web_search": "web_search",
     "websearch": "web_search",
+    "google_search": "web_search",
+    "google_search_retrieval": "web_search",
+    "google_search_grounding": "web_search",
     "web_fetch": "web_fetch",
     "webfetch": "web_fetch",
     "fetch_url": "web_fetch",

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":