test: cover the pre-session security check + fix pyright annotation

- Add ``test_bad_host_header_rejected_before_session_allocation`` that
  drives the DNS-rebinding branch of the manager and asserts the
  request is rejected with 421 without allocating a session. Restores
  the strict 100% coverage on ``streamable_http_manager.py``.
- Suppress the pyright ``reportUnknownMemberType`` on
  ``manager._task_group._tasks`` — that attribute is private anyio
  internals with no exported type, but we deliberately introspect it to
  prove the leaked task from the original bug is no longer spawned.

Author: raphaelOhana

tests/server/test_streamable_http_manager.py

@@ -14,6 +14,7 @@
 from mcp.server.lowlevel import Server
 from mcp.server.streamable_http import MCP_SESSION_ID_HEADER, StreamableHTTPServerTransport
 from mcp.server.streamable_http_manager import StreamableHTTPSessionManager
+from mcp.server.transport_security import TransportSecuritySettings
 from mcp.types import INVALID_REQUEST
 
 
@@ -435,9 +436,8 @@ async def mock_receive():  # pragma: no cover
         )
         assert manager._task_group is not None
         # anyio TaskGroup internals: no live tasks belonging to run_server
-        assert len(manager._task_group._tasks) == 0, (
-            f"{method} without session-id must not spawn a background task — leaked {len(manager._task_group._tasks)}"
-        )
+        live_tasks = len(manager._task_group._tasks)  # type: ignore[attr-defined]
+        assert live_tasks == 0, f"{method} without session-id must not spawn a background task — leaked {live_tasks}"
 
         # Response should be a well-formed JSON-RPC error at status 400
         response_start = next(
@@ -453,6 +453,62 @@ async def mock_receive():  # pragma: no cover
         assert "Missing session ID" in error_data["error"]["message"]
 
 
+@pytest.mark.anyio
+async def test_bad_host_header_rejected_before_session_allocation():
+    """Security check runs before session allocation.
+
+    With DNS-rebinding protection enabled, a request that presents a
+    Host header not in the allow-list must be rejected with 421 without
+    allocating a session. Previously this check lived only in the
+    transport, so a bad-Host request would allocate a session first and
+    then get rejected — the allocated session and its task were leaked.
+    """
+    app = Server("test-bad-host")
+    manager = StreamableHTTPSessionManager(
+        app=app,
+        security_settings=TransportSecuritySettings(
+            enable_dns_rebinding_protection=True, allowed_hosts=["127.0.0.1"]
+        ),
+    )
+
+    async with manager.run():
+        sent_messages: list[Message] = []
+
+        async def mock_send(message: Message):
+            sent_messages.append(message)
+
+        scope: Scope = {
+            "type": "http",
+            "method": "POST",
+            "path": "/mcp",
+            "headers": [
+                (b"host", b"evil.com"),
+                (b"content-type", b"application/json"),
+                (b"accept", b"application/json, text/event-stream"),
+            ],
+        }
+
+        async def mock_receive():  # pragma: no cover
+            return {"type": "http.request", "body": b"{}", "more_body": False}
+
+        assert len(manager._server_instances) == 0
+
+        await manager.handle_request(scope, mock_receive, mock_send)
+
+        # Session must NOT have been allocated
+        assert len(manager._server_instances) == 0, (
+            "Bad-Host request must not allocate a session (was rejected by security check)"
+        )
+
+        # And the response must be the 421 the middleware produced
+        response_start = next(
+            (msg for msg in sent_messages if msg["type"] == "http.response.start"),
+            None,
+        )
+        assert response_start is not None
+        assert response_start["status"] == 421
+
+
 def test_session_idle_timeout_rejects_non_positive():
     with pytest.raises(ValueError, match="positive number"):
         StreamableHTTPSessionManager(app=Server("test"), session_idle_timeout=-1)