fix: add validation tests and fix formatting for idle timeout

Github-Issue: #1283

Author: felixweinberger

tests/issues/test_1283_idle_session_cleanup.py

@@ -159,6 +159,24 @@ async def test_multiple_sessions_reaped_independently():
         stop.set()
 
 
+def test_session_idle_timeout_rejects_negative():
+    """session_idle_timeout must be a positive number."""
+    with pytest.raises(ValueError, match="positive number"):
+        StreamableHTTPSessionManager(app=Server("test"), session_idle_timeout=-1)
+
+
+def test_session_idle_timeout_rejects_zero():
+    """session_idle_timeout must be a positive number."""
+    with pytest.raises(ValueError, match="positive number"):
+        StreamableHTTPSessionManager(app=Server("test"), session_idle_timeout=0)
+
+
+def test_session_idle_timeout_rejects_stateless():
+    """session_idle_timeout is not supported in stateless mode."""
+    with pytest.raises(ValueError, match="not supported in stateless"):
+        StreamableHTTPSessionManager(app=Server("test"), session_idle_timeout=30, stateless=True)
+
+
 @pytest.mark.anyio
 async def test_terminate_idempotency():
     """Calling terminate() multiple times should be safe."""
@@ -303,8 +321,7 @@ async def instrumented_run(*args, **kwargs):  # type: ignore[no-untyped-def]
         # should exit almost instantly after terminate().  We allow up to 2x
         # the idle timeout as a generous upper bound to avoid flakiness.
         assert total_elapsed < idle_timeout * 3, (
-            f"run_server task took {total_elapsed:.3f}s to exit after pre_reap_time; "
-            f"expected < {idle_timeout * 3:.1f}s"
+            f"run_server task took {total_elapsed:.3f}s to exit after pre_reap_time; expected < {idle_timeout * 3:.1f}s"
         )
 
         # The session must have been removed from tracking dicts.