refactor(telemetry): simplify by eliminating TelemetryFacade

**Problem:**
The telemetry system had unnecessary abstraction layers and bad practices
from the last 3 commits introducing the gateway implementation:
- TelemetryFacade class wrapper around emit() function
- String literals instead of SignalType enum
- Dictionary mapping enum → string instead of enum → enum
- Unnecessary ENTERPRISE_TELEMETRY_GATEWAY_ENABLED feature flag
- Duplicate guard checks scattered across files
- Non-thread-safe TelemetryGateway singleton pattern
- Missing guard in ops_trace_task.py causing RuntimeError spam

**Solution:**
1. Deleted TelemetryFacade - replaced with thin emit() function in core/telemetry/__init__.py
2. Added SignalType enum ('trace' | 'metric_log') to enterprise/telemetry/contracts.py
3. Replaced CASE_TO_TRACE_TASK_NAME dict with CASE_TO_TRACE_TASK: dict[TelemetryCase, TraceTaskName]
4. Deleted is_gateway_enabled() and _emit_legacy() - using existing ENTERPRISE_ENABLED + ENTERPRISE_TELEMETRY_ENABLED instead
5. Extracted _should_drop_ee_only_event() helper to eliminate duplicate checks
6. Moved TelemetryGateway singleton to ext_enterprise_telemetry.py:
   - Init once in init_app() for thread-safety
   - Access via get_gateway() function
7. Re-added guard to ops_trace_task.py to prevent RuntimeError when EE=OFF but CE tracing enabled
8. Updated 11 caller files to import 'emit as telemetry_emit' instead of 'TelemetryFacade'

**Result:**
- 322 net lines deleted (533 removed, 211 added)
- All 91 tests pass
- Thread-safe singleton pattern
- Cleaner API surface: from TelemetryFacade.emit() to telemetry_emit()
- Proper enum usage throughout
- No RuntimeError spam in EE=OFF + CE=ON scenario

api/extensions/ext_enterprise_telemetry.py

@@ -1,9 +1,11 @@
 """Flask extension for enterprise telemetry lifecycle management.
 
-Initializes the EnterpriseExporter singleton during ``create_app()`` (single-threaded),
-registers blinker event handlers, and hooks atexit for graceful shutdown.
+Initializes the EnterpriseExporter and TelemetryGateway singletons during
+``create_app()`` (single-threaded), registers blinker event handlers,
+and hooks atexit for graceful shutdown.
 
-Skipped entirely when ``ENTERPRISE_ENABLED`` and ``ENTERPRISE_TELEMETRY_ENABLED`` are false (``is_enabled()`` gate).
+Skipped entirely when ``ENTERPRISE_ENABLED`` and ``ENTERPRISE_TELEMETRY_ENABLED``
+are false (``is_enabled()`` gate).
 """
 
 from __future__ import annotations
@@ -17,10 +19,12 @@ from configs import dify_config
 if TYPE_CHECKING:
     from dify_app import DifyApp
     from enterprise.telemetry.exporter import EnterpriseExporter
+    from enterprise.telemetry.gateway import TelemetryGateway
 
 logger = logging.getLogger(__name__)
 
 _exporter: EnterpriseExporter | None = None
+_gateway: TelemetryGateway | None = None
 
 
 def is_enabled() -> bool:
@@ -28,14 +32,16 @@ def is_enabled() -> bool:
 
 
 def init_app(app: DifyApp) -> None:
-    global _exporter
+    global _exporter, _gateway
 
     if not is_enabled():
         return
 
     from enterprise.telemetry.exporter import EnterpriseExporter
+    from enterprise.telemetry.gateway import TelemetryGateway
 
     _exporter = EnterpriseExporter(dify_config)
+    _gateway = TelemetryGateway()
     atexit.register(_exporter.shutdown)
 
     # Import to trigger @signal.connect decorator registration
@@ -46,3 +52,7 @@ def init_app(app: DifyApp) -> None:
 
 def get_enterprise_exporter() -> EnterpriseExporter | None:
     return _exporter
+
+
+def get_gateway() -> TelemetryGateway | None:
+    return _gateway