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/file/runtime.py

@@ -0,0 +1,57 @@
+from __future__ import annotations
+
+from collections.abc import Generator
+
+from .protocols import HttpResponseProtocol, WorkflowFileRuntimeProtocol
+
+
+class WorkflowFileRuntimeNotConfiguredError(RuntimeError):
+    """Raised when workflow file runtime dependencies were not configured."""
+
+
+class _UnconfiguredWorkflowFileRuntime(WorkflowFileRuntimeProtocol):
+    def _raise(self) -> None:
+        raise WorkflowFileRuntimeNotConfiguredError(
+            "workflow file runtime is not configured, call set_workflow_file_runtime(...) first"
+        )
+
+    @property
+    def files_url(self) -> str:
+        self._raise()
+
+    @property
+    def internal_files_url(self) -> str | None:
+        self._raise()
+
+    @property
+    def secret_key(self) -> str:
+        self._raise()
+
+    @property
+    def files_access_timeout(self) -> int:
+        self._raise()
+
+    @property
+    def multimodal_send_format(self) -> str:
+        self._raise()
+
+    def http_get(self, url: str, *, follow_redirects: bool = True) -> HttpResponseProtocol:
+        self._raise()
+
+    def storage_load(self, path: str, *, stream: bool = False) -> bytes | Generator:
+        self._raise()
+
+    def sign_tool_file(self, *, tool_file_id: str, extension: str, for_external: bool = True) -> str:
+        self._raise()
+
+
+_runtime: WorkflowFileRuntimeProtocol = _UnconfiguredWorkflowFileRuntime()
+
+
+def set_workflow_file_runtime(runtime: WorkflowFileRuntimeProtocol) -> None:
+    global _runtime
+    _runtime = runtime
+
+
+def get_workflow_file_runtime() -> WorkflowFileRuntimeProtocol:
+    return _runtime