test: fix MCP unit tests and add webui_mcp flag support
- Add SKIP_MODEL_PRELOAD env var to conftest.py for tests using local models
- Add webui_mcp option to ServerProcess class in utils.py
- Fix test_mcp.py to use local model instead of HuggingFace download
- Fix Python echo script to ignore JSON-RPC notifications (no response for notifications)
- Fix test_mcp_servers_with_config assertion (API returns list of objects not strings)
- Fix test_websocket_connection_without_server_param to handle empty response
- Fix test_websocket_connection_invalid_server to send message first
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
