refactor(workflow-file): phase2 replace remaining imports and remove core.file

Complete phase 2 of the file module migration by replacing the remaining repository-wide legacy imports and deleting the temporary core.file compatibility package introduced in phase 1.

What this commit changes
- Replace legacy core.file.* imports with core.workflow.file.* across:
  - controllers
  - core app/agent/datasource/prompt/rag/tools/variables
  - factories, fields, libs, models, services
  - otel parser integration points
  - unit and integration tests that referenced legacy paths
- Migrate residual runtime usages in app/task pipeline paths that still referenced core.file symbols.
- Update tests and model serialization helpers that relied on old module paths.
- Remove the compatibility bridge package entirely:
  - delete core/file/__init__.py
  - delete core/file/constants.py
  - delete core/file/enums.py
  - delete core/file/file_manager.py
  - delete core/file/helpers.py
  - delete core/file/models.py
  - delete core/file/tool_file_parser.py

Verification
- No Python references to core.file remain ( -> empty).
- Targeted regression tests for migrated file primitives and factory/type flows passed:
  - tests/unit_tests/core/test_file.py
  - tests/unit_tests/factories/test_variable_factory.py
  - tests/unit_tests/services/test_variable_truncator.py

Result
- The repository now uses core.workflow.file as the single canonical file namespace.
- The migration is fully split into two commits: phase 1 compatibility + phase 2 full cutover.

api/tests/unit_tests/services/test_variable_truncator.py

@@ -17,8 +17,6 @@ from uuid import uuid4
 
 import pytest
 
-from core.file.enums import FileTransferMethod, FileType
-from core.file.models import File
 from core.variables.segments import (
     ArrayFileSegment,
     ArrayNumberSegment,
@@ -30,6 +28,8 @@ from core.variables.segments import (
     ObjectSegment,
     StringSegment,
 )
+from core.workflow.file.enums import FileTransferMethod, FileType
+from core.workflow.file.models import File
 from services.variable_truncator import (
     DummyVariableTruncator,
     MaxDepthExceededError,