Config push notify pattern: replace stateful pub/sub with signal+ fetch (#760)

Replace the config push mechanism that broadcast the full config
blob on a 'state' class pub/sub queue with a lightweight notify
signal containing only the version number and affected config
types. Processors fetch the full config via request/response from
the config service when notified.

This eliminates the need for the pub/sub 'state' queue class and
stateful pub/sub services entirely. The config push queue moves
from 'state' to 'flow' class — a simple transient signal rather
than a retained message.  This solves the RabbitMQ
late-subscriber problem where restarting processes never received
the current config because their fresh queue had no historical
messages.

Key changes:
- ConfigPush schema: config dict replaced with types list
- Subscribe-then-fetch startup with retry: processors subscribe
  to notify queue, fetch config via request/response, then
  process buffered notifies with version comparison to avoid race
  conditions
- register_config_handler() accepts optional types parameter so
  handlers only fire when their config types change
- Short-lived config request/response clients to avoid subscriber
  contention on non-persistent response topics
- Config service passes affected types through put/delete/flow
  operations
- Gateway ConfigReceiver rewritten with same notify pattern and
  retry loop

Tests updated

New tests:
- register_config_handler: without types, with types, multiple
  types, multiple handlers
- on_config_notify: old/same version skipped, irrelevant types
  skipped (version still updated), relevant type triggers fetch,
  handler without types always called, mixed handler filtering,
  empty types invokes all, fetch failure handled gracefully
- fetch_config: returns config+version, raises on error response,
  stops client even on exception
- fetch_and_apply_config: applies to all handlers on startup,
  retries on failure

tests/unit/test_base/test_flow_processor.py

@@ -35,7 +35,9 @@ class TestFlowProcessorSimple(IsolatedAsyncioTestCase):
         mock_async_init.assert_called_once()
         
         # Verify register_config_handler was called with the correct handler
-        mock_register_config.assert_called_once_with(processor.on_configure_flows)
+        mock_register_config.assert_called_once_with(
+            processor.on_configure_flows, types=["active-flow"]
+        )
         
         # Verify FlowProcessor-specific initialization
         assert hasattr(processor, 'flows')