UN-3211 [FEAT] HTTP session lifecycle management for workers API clients

- Add _owns_session flag to prevent singleton shared session from being
  closed by individual clients
- Wire API_CLIENT_POOL_SIZE into HTTPAdapter connection pools
- Add idempotent close() and __del__ destructor to BaseAPIClient
- Add try/finally cleanup in api-deployment and callback tasks
- Add on_worker_process_shutdown hook and early-return guard in postrun
- Add 25 unit tests for session lifecycle behavior

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: muhammad-ali-e

workers/api-deployment/tasks.py

@@ -168,10 +168,11 @@ def _unified_api_execution(
     Returns:
         Execution result dictionary
     """
+    api_client = None
     try:
         # Set up execution context using shared utilities
         organization_id = schema_name
-        config, api_client = WorkerExecutionContext.setup_execution_context(
+        _, api_client = WorkerExecutionContext.setup_execution_context(
             organization_id, execution_id, workflow_id
         )
 
@@ -233,22 +234,13 @@ def _unified_api_execution(
             f"files_processed={len(converted_files)}",
         )
 
-        # CRITICAL: Clean up StateStore to prevent data leaks between tasks
-        try:
-            from shared.infrastructure.context import StateStore
-
-            StateStore.clear_all()
-            logger.debug("🧹 Cleaned up StateStore context to prevent data leaks")
-        except Exception as cleanup_error:
-            logger.warning(f"Failed to cleanup StateStore context: {cleanup_error}")
-
         return result
 
     except Exception as e:
         logger.error(f"API execution failed: {e}")
 
         # Handle execution error with standardized pattern
-        if "api_client" in locals():
+        if api_client is not None:
             WorkerExecutionContext.handle_execution_error(
                 api_client, execution_id, e, logger, f"api_execution_{task_type}"
             )
@@ -261,26 +253,29 @@ def _unified_api_execution(
             f"error={str(e)}",
         )
 
-        # CRITICAL: Clean up StateStore to prevent data leaks between tasks (error path)
-        try:
-            from shared.infrastructure.context import StateStore
-
-            StateStore.clear_all()
-            logger.debug(
-                "🧹 Cleaned up StateStore context to prevent data leaks (error path)"
-            )
-        except Exception as cleanup_error:
-            logger.warning(
-                f"Failed to cleanup StateStore context on error: {cleanup_error}"
-            )
-
         return {
             "execution_id": execution_id,
             "status": "ERROR",
             "error": str(e),
             "files_processed": 0,
         }
 
+    finally:
+        # Clean up API client session to prevent socket FD leaks
+        if api_client is not None:
+            try:
+                api_client.close()
+            except Exception:
+                pass
+
+        # Clean up StateStore to prevent data leaks between tasks
+        try:
+            from shared.infrastructure.context import StateStore
+
+            StateStore.clear_all()
+        except Exception as cleanup_error:
+            logger.warning(f"Failed to cleanup StateStore context: {cleanup_error}")
+
 
 @app.task(
     bind=True,