Add native Sliding Sync (MSC3575) endpoint to worker routing

The /_matrix/client/unstable/org.matrix.simplified_msc3575/sync endpoint
can be handled by generic workers, but Synapse's workers.md documentation
doesn't mention it. The code confirms it's worker-compatible:

- SlidingSyncRestServlet is registered via sync.register_servlets:
  https://github.com/element-hq/synapse/blob/0dfcffab0f/synapse/rest/client/sync.py#L1128-L1131

- sync.register_servlets is NOT in the worker exclusion list:
  https://github.com/element-hq/synapse/blob/0dfcffab0f/synapse/rest/__init__.py#L180-L194

- GenericWorkerStore includes SlidingSyncStore:
  https://github.com/element-hq/synapse/blob/0dfcffab0f/synapse/app/generic_worker.py#L168

This adds the endpoint to both:
- matrix_synapse_workers_sync_worker_client_server_endpoints (for specialized sync workers with sticky routing)
- matrix_synapse_workers_generic_worker_endpoints (documenting generic worker capability)

roles/custom/matrix-synapse/vars/main.yml

@@ -26,6 +26,8 @@ matrix_synapse_workers_room_worker_federation_endpoints:
 # Sync workers handle /sync and the (now deprecated) related endpoints
 matrix_synapse_workers_sync_worker_client_server_endpoints:
   - ^/_matrix/client/(api/v1|r0|v3|unstable)/(sync|events|initialSync|rooms/[^/]+/initialSync)$
+  # Native Sliding Sync (MSC3575) - supported on generic workers since Synapse 1.114
+  - ^/_matrix/client/unstable/org.matrix.simplified_msc3575/sync$
 
 # Client reader workers handle generic client-server endpoints that don't contain a roomid or sync
 matrix_synapse_workers_client_reader_client_server_endpoints:
@@ -149,6 +151,8 @@ matrix_synapse_workers_generic_worker_endpoints:
   - ^/_matrix/client/(api/v1|r0|v3)/events$
   - ^/_matrix/client/(api/v1|r0|v3)/initialSync$
   - ^/_matrix/client/(api/v1|r0|v3)/rooms/[^/]+/initialSync$
+  # Native Sliding Sync (MSC3575) - supported since Synapse 1.114
+  - ^/_matrix/client/unstable/org.matrix.simplified_msc3575/sync$
 
   # Federation requests
   - ^/_matrix/federation/v1/event/