test(matrix): set user_id in approval-reaction test to bypass defensive self-drop

MatrixAdapter._is_self_sender returns True defensively when _user_id is empty
(whoami not yet resolved) to prevent echo loops — see #15763. The reaction
approval test must therefore initialize a user_id so _on_reaction does not
drop the inbound test event before reaching the approval handler.

tests/gateway/test_matrix_exec_approval.py

@@ -37,6 +37,8 @@ class TestMatrixExecApprovalReactions:
         from gateway.platforms.matrix import MatrixAdapter, _MatrixApprovalPrompt
 
         adapter = MatrixAdapter(PlatformConfig(enabled=True, token="tok", extra={"homeserver": "https://matrix.example.org"}))
+        # Resolve user_id so _is_self_sender doesn't defensively drop all traffic (#15763).
+        adapter._user_id = "@bot:example.org"
         adapter._approval_prompts_by_event["$target"] = _MatrixApprovalPrompt(
             session_key="sess-1", chat_id="!room:example.org", message_id="$target"
         )