refactor(telemetry): move gateway to core as stateless module-level functions

Move routing table, emit(), and is_enterprise_telemetry_enabled() from
enterprise/telemetry/gateway.py into core/telemetry/gateway.py so both
CE and EE share one code path. The ce_eligible flag in CASE_ROUTING
controls which events flow in CE — flipping it is the only change needed
to enable an event in community edition.

- Delete enterprise/telemetry/gateway.py (class-based singleton)
- Create core/telemetry/gateway.py (stateless functions, no shared state)
- Simplify core/telemetry/__init__.py to thin facade over gateway
- Remove TelemetryGateway class and get_gateway() from ext_enterprise_telemetry
- Single-source is_enterprise_telemetry_enabled in core.telemetry.gateway
- Fix pre-existing test bugs (missing dify.event.id in metric handler tests)
- Update all imports and mock paths across 7 test files

api/core/ops/ops_trace_manager.py

@@ -1285,7 +1285,7 @@ class TraceQueueManager:
         self.trace_instance = OpsTraceManager.get_ops_trace_instance(app_id)
         self.flask_app = current_app._get_current_object()  # type: ignore
 
-        from core.telemetry import is_enterprise_telemetry_enabled
+        from core.telemetry.gateway import is_enterprise_telemetry_enabled
 
         self._enterprise_telemetry_enabled = is_enterprise_telemetry_enabled()
         if trace_manager_timer is None: