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/core/rag/index_processor/processor/paragraph_index_processor.py

@@ -9,7 +9,6 @@ from typing import Any, cast
 logger = logging.getLogger(__name__)
 
 from core.entities.knowledge_entities import PreviewDetail
-from core.file import File, FileTransferMethod, FileType, file_manager
 from core.llm_generator.prompts import DEFAULT_GENERATOR_SUMMARY_PROMPT
 from core.model_manager import ModelInstance
 from core.model_runtime.entities.llm_entities import LLMResult, LLMUsage
@@ -35,6 +34,7 @@ from core.rag.index_processor.index_processor_base import BaseIndexProcessor
 from core.rag.models.document import AttachmentDocument, Document, MultimodalGeneralStructureChunk
 from core.rag.retrieval.retrieval_methods import RetrievalMethod
 from core.tools.utils.text_processing_utils import remove_leading_symbols
+from core.workflow.file import File, FileTransferMethod, FileType, file_manager
 from core.workflow.nodes.llm import llm_utils
 from extensions.ext_database import db
 from factories.file_factory import build_from_mapping