simplify(compat): gateway — re-land 92d0bd0d73 (reverted by stale-index commit b818085298)

Re-applies the gateway compat removal byte-for-byte; see 92d0bd0d73 for the
full inventory (30 re-exports/aliases + 2 shim modules dropped, 3 shim-only
names re-removed, 24 callers + 34 test files repointed). No new changes.

tests/gateway/test_busy_session_ack.py

@@ -208,7 +208,7 @@ class TestBusySessionAck:
         agent.steer = MagicMock(return_value=True)
         runner._running_agents[sk] = agent
 
-        with patch("gateway.run.merge_pending_message_event") as mock_merge:
+        with patch("gateway.platforms.base.merge_pending_message_event") as mock_merge:
             await runner._handle_active_session_busy_message(event, sk)
 
         # VERIFY: Agent was steered, NOT interrupted