Handle OSError when validating registered project paths on startup

When a registered project path points to a stale or unreachable
filesystem (e.g. a disconnected SSHFS mount), `path.exists()` raises
`OSError: [Errno 5] Input/output error` instead of returning False.
This uncaught exception crashes the entire MCP server on startup,
making all Serena tools unavailable.

Wrap the `path.exists()` call in a try/except OSError so that
inaccessible paths are gracefully skipped with a warning, matching
the existing behavior for non-existent paths.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/serena/config/serena_config.py

@@ -826,7 +826,12 @@ class SerenaConfig(SharedConfig):
         instance.projects = []
         for path in loaded_commented_yaml["projects"]:
             path = Path(path).resolve()
-            if not path.exists() or (path.is_dir() and not os.path.isfile(instance.get_project_yml_location(str(path)))):
+            try:
+                path_exists = path.exists()
+            except OSError as e:
+                log.warning(f"Project path {path} is not accessible ({e}), skipping.")
+                continue
+            if not path_exists or (path.is_dir() and not os.path.isfile(instance.get_project_yml_location(str(path)))):
                 log.warning(f"Project path {path} does not exist or no associated project configuration file found, skipping.")
                 continue
             if path.is_file():