fix: restore pragma no cover on unreachable cancel_scope guard

Line 147-148 is unreachable: _cancel_scope is always set in __init__.
The previous pragma removal was too aggressive - only remove pragmas
from lines that are actually covered by the new tests.

Author: dragogargo

src/mcp/shared/session.py

@@ -144,7 +144,7 @@ async def cancel(self) -> None:
         """Cancel this request and mark it as completed."""
         if not self._entered:
             raise RuntimeError("RequestResponder must be used as a context manager")
-        if not self._cancel_scope:
+        if not self._cancel_scope:  # pragma: no cover
             raise RuntimeError("No active cancel scope")
 
         self._cancel_scope.cancel()