feat(mcp): add MCP server log panel backend (#2308)

- Add log buffer (_log_buffer: deque with maxlen=500) to RuntimeMCPSession
- Capture stderr from Box managed process in monitor_process_health loop
- Add get_mcp_server_logs service method with limit and level filtering
- Add GET /servers/<server_name>/logs HTTP endpoint
- Parse log level from stderr lines (error/warning/debug/info)
- Tests passing: 211 passed, 12 skipped

Co-authored-by: dadachann <185672915+dadachann@users.noreply.github.com>

src/langbot/pkg/provider/tools/loaders/mcp.py

@@ -268,6 +268,13 @@ class RuntimeMCPSession:
         # process (it will be re-attached on the next initialize()).
         self._preserve_managed_process = False
 
+        # Log buffer for capturing stderr from Box managed process (maxlen=500 keeps
+        # recent lines without unbounded memory growth)
+        import collections as _collections
+
+        self._log_buffer: _collections.deque = _collections.deque(maxlen=500)
+        self._last_stderr_text: str = ''
+
         self._box_stdio_runtime = BoxStdioSessionRuntime(self)
         self.box_config = self._box_stdio_runtime.config