refactor(workflow-file): phase1 migrate workflow file deps with compatibility bridge

Implement phase 1 of the file module migration by moving workflow-facing file primitives to core.workflow.file while keeping core.file as a temporary compatibility layer.

What this commit changes
- Add core.workflow.file package (constants/enums/models/helpers/file_manager/tool_file_parser).
- Add protocol-based runtime binding in core.workflow.file.runtime and core.workflow.file.protocols.
- Add application adapter core.app.workflow.file_runtime and bind runtime in extensions.ext_storage.init_app.
- Bind runtime in tests via tests/conftest.py.
- Migrate workflow-only imports from core.file.* to core.workflow.file.* across workflow runtime/nodes/entry/encoder and workflow node factory.
- Update workflow unit tests to patch/import the new workflow file namespace.
- Remove workflow-external-imports ignore_imports entries related to core.file from .importlinter.

Compatibility guarantee for phase split
- Keep core.file import path available in this phase by replacing core/file/*.py with forwarding bridge modules that re-export core.workflow.file symbols.
- Preserve runtime behavior and isinstance(File) identity consistency while non-workflow modules are still on legacy import paths.

Notes
- This commit intentionally does not remove core.file. Full repository replacement and bridge removal are handled in phase 2.

api/core/workflow/runtime/variable_pool.py

@@ -8,7 +8,6 @@ from typing import Annotated, Any, Union, cast
 
 from pydantic import BaseModel, Field
 
-from core.file import File, FileAttribute, file_manager
 from core.variables import Segment, SegmentGroup, VariableBase
 from core.variables.consts import SELECTORS_LENGTH
 from core.variables.segments import FileSegment, ObjectSegment
@@ -19,6 +18,7 @@ from core.workflow.constants import (
     RAG_PIPELINE_VARIABLE_NODE_ID,
     SYSTEM_VARIABLE_NODE_ID,
 )
+from core.workflow.file import File, FileAttribute, file_manager
 from core.workflow.system_variable import SystemVariable
 from factories import variable_factory