Implemented validation for session_id at the very start of A2aLocalFacade._handle_session_close, prior to evaluating svc is None, ensuring that empty session IDs never reach _cleanup_session_devcontainers on any code path.

Updated features/a2a_facade_coverage.feature to reflect the new behavior (empty session_id now raises error in both branches).

Added new features/a2a_session_close_validation.feature with 6 BDD scenarios covering all branches.

Added new features/steps/a2a_session_close_validation_steps.py with step definitions.

ISSUES CLOSED: #9094

@@ -0,0 +1,49 @@
+Feature: A2A session close session_id validation — issue #9094
+  @a2a @session

@@ -0,0 +82,4 @@
+def session_close_valid_no_service() -> None:
+    """Valid session_id with no service → should return closed (ok)."""
+    facade = A2aLocalFacade()
+    request = A2aRequest(method="session.close", params={"session_id": "valid-session-robot"})

@@ -0,0 +133,4 @@
+def session_close_valid_wired_service() -> None:
+    """Valid session_id with wired service → should return closed (ok)."""
+    facade = A2aLocalFacade(services={"session_service": _build_mock_session_service()})
+    request = A2aRequest(method="session.close", params={"session_id": "valid-session-robot"})

@@ -0,0 +82,4 @@
+def session_close_valid_no_service() -> None:
+    """Valid session_id with no service → should return closed (ok)."""
+    facade = A2aLocalFacade()
+    request = A2aRequest(method="session.close", params={"session_id": "valid-session-robot"})