fix: ensure 100% branch coverage for __aexit__ lifecycle transitions

Add tests for __aexit__ when session is already in Closing or Closed
state, covering the 'if' False branch and the second except RuntimeError.
Mark the first except RuntimeError as pragma: no cover since it is
unreachable under the current transition table (all non-terminal states
can transition to Closing).

Author: dgenio

src/mcp/server/session.py

@@ -267,7 +267,7 @@ async def __aexit__(
                 InitializationState.Closing,
             ):
                 self._transition_state(InitializationState.Closing)
-        except RuntimeError:
+        except RuntimeError:  # pragma: no cover
             logger.debug("Could not transition to Closing from %s", self._initialization_state.name)
         try:
             return await super().__aexit__(exc_type, exc_val, exc_tb)

tests/server/test_session_lifecycle.py

@@ -220,6 +220,25 @@ async def test_aexit_stateless_transitions_to_closed() -> None:
         assert session.initialization_state == InitializationState.Closed
 
 
+async def test_aexit_already_closing() -> None:
+    """__aexit__ skips Closing transition when already in Closing state."""
+    async with _session_context() as session:
+        async with session:
+            session._transition_state(InitializationState.Closing)
+
+        assert session.initialization_state == InitializationState.Closed
+
+
+async def test_aexit_already_closed() -> None:
+    """__aexit__ handles already-Closed sessions gracefully."""
+    async with _session_context() as session:
+        async with session:
+            session._transition_state(InitializationState.Closing)
+            session._transition_state(InitializationState.Closed)
+
+        assert session.initialization_state == InitializationState.Closed
+
+
 # ---------------------------------------------------------------------------
 # Integration: full handshake lifecycle
 # ---------------------------------------------------------------------------