fix: update all test mocks for call_llm migration

Update 14 test files to use the new call_llm/async_call_llm mock
patterns instead of the old get_text_auxiliary_client/
get_vision_auxiliary_client tuple returns.

- vision_tools tests: mock async_call_llm instead of _aux_async_client
- browser tests: mock call_llm instead of _aux_vision_client
- flush_memories tests: mock call_llm instead of get_text_auxiliary_client
- session_search tests: mock async_call_llm with RuntimeError
- mcp_tool tests: fix whitelist model config, use side_effect for
  multi-response tests
- auxiliary_config_bridge: update for model=None (resolved in router)

3251 passed, 2 pre-existing unrelated failures.

tests/test_auxiliary_config_bridge.py

@@ -229,13 +229,14 @@ class TestVisionModelOverride:
 
     def test_default_model_when_no_override(self, monkeypatch):
         monkeypatch.delenv("AUXILIARY_VISION_MODEL", raising=False)
-        from tools.vision_tools import _handle_vision_analyze, DEFAULT_VISION_MODEL
+        from tools.vision_tools import _handle_vision_analyze
         with patch("tools.vision_tools.vision_analyze_tool", new_callable=MagicMock) as mock_tool:
             mock_tool.return_value = '{"success": true}'
             _handle_vision_analyze({"image_url": "http://test.jpg", "question": "test"})
             call_args = mock_tool.call_args
-            expected = DEFAULT_VISION_MODEL or "google/gemini-3-flash-preview"
-            assert call_args[0][2] == expected
+            # With no AUXILIARY_VISION_MODEL env var, model should be None
+            # (the centralized call_llm router picks the provider default)
+            assert call_args[0][2] is None
 
 
 # ── DEFAULT_CONFIG shape tests ───────────────────────────────────────────────