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/core/app/apps/advanced_chat/generate_task_pipeline.py

@@ -63,7 +63,8 @@ from core.base.tts import AppGeneratorTTSPublisher, AudioTrunk
 from core.model_runtime.entities.llm_entities import LLMUsage
 from core.model_runtime.utils.encoders import jsonable_encoder
 from core.ops.ops_trace_manager import TraceQueueManager
-from core.telemetry import TelemetryContext, TelemetryEvent, TelemetryFacade, TraceTaskName
+from core.telemetry import TelemetryContext, TelemetryEvent, TraceTaskName
+from core.telemetry import emit as telemetry_emit
 from core.workflow.enums import WorkflowExecutionStatus
 from core.workflow.nodes import NodeType
 from core.workflow.repositories.draft_variable_repository import DraftVariableSaverFactory
@@ -832,7 +833,7 @@ class AdvancedChatAppGenerateTaskPipeline(GraphRuntimeStateSupport):
         session.add_all(message_files)
 
         if trace_manager:
-            TelemetryFacade.emit(
+            telemetry_emit(
                 TelemetryEvent(
                     name=TraceTaskName.MESSAGE_TRACE,
                     context=TelemetryContext(

api/core/app/task_pipeline/easy_ui_based_generate_task_pipeline.py

@@ -55,7 +55,8 @@ from core.model_runtime.model_providers.__base.large_language_model import Large
 from core.ops.ops_trace_manager import TraceQueueManager
 from core.prompt.utils.prompt_message_util import PromptMessageUtil
 from core.prompt.utils.prompt_template_parser import PromptTemplateParser
-from core.telemetry import TelemetryContext, TelemetryEvent, TelemetryFacade, TraceTaskName
+from core.telemetry import TelemetryContext, TelemetryEvent, TraceTaskName
+from core.telemetry import emit as telemetry_emit
 from events.message_event import message_was_created
 from extensions.ext_database import db
 from libs.datetime_utils import naive_utc_now
@@ -409,7 +410,7 @@ class EasyUIBasedGenerateTaskPipeline(BasedGenerateTaskPipeline):
         message.message_metadata = self._task_state.metadata.model_dump_json()
 
         if trace_manager:
-            TelemetryFacade.emit(
+            telemetry_emit(
                 TelemetryEvent(
                     name=TraceTaskName.MESSAGE_TRACE,
                     context=TelemetryContext(

api/core/app/workflow/layers/persistence.py

@@ -395,9 +395,10 @@ class WorkflowPersistenceLayer(GraphEngineLayer):
             external_trace_id = self._application_generate_entity.extras.get("external_trace_id")
             parent_trace_context = self._application_generate_entity.extras.get("parent_trace_context")
 
-        from core.telemetry import TelemetryContext, TelemetryEvent, TelemetryFacade, TraceTaskName
+        from core.telemetry import TelemetryContext, TelemetryEvent, TraceTaskName
+        from core.telemetry import emit as telemetry_emit
 
-        TelemetryFacade.emit(
+        telemetry_emit(
             TelemetryEvent(
                 name=TraceTaskName.WORKFLOW_TRACE,
                 context=TelemetryContext(
@@ -499,9 +500,10 @@ class WorkflowPersistenceLayer(GraphEngineLayer):
         if parent_trace_context:
             node_data["parent_trace_context"] = parent_trace_context
 
-        from core.telemetry import TelemetryContext, TelemetryEvent, TelemetryFacade, TraceTaskName
+        from core.telemetry import TelemetryContext, TelemetryEvent, TraceTaskName
+        from core.telemetry import emit as telemetry_emit
 
-        TelemetryFacade.emit(
+        telemetry_emit(
             TelemetryEvent(
                 name=TraceTaskName.NODE_EXECUTION_TRACE,
                 context=TelemetryContext(