Bugfix: Pydantic deserialization for FlyteFile and FlyteDirectory

[gverkes]

Fixes #6669

Tracking issue

Closes flyteorg/flyte#6669

Why are the changes needed?

When deserializing Pydantic models containing FlyteFile or FlyteDirectory fields using model_validate(), the deserialized objects were
missing private attributes (_remote_source, _downloader, etc.). This caused an AttributeError when attempting to re-serialize these
objects with model_dump(), breaking the serialize → deserialize → serialize cycle.

This is a critical bug that prevents users from using FlyteFile/FlyteDirectory within Pydantic BaseModel classes in a normal way.

What changes were proposed in this pull request?

Added Pydantic model validators to both FlyteFile and FlyteDirectory classes that ensure private attributes are properly initialized during
deserialization:

1. FlyteFile (flytekit/types/file/file.py):

   • Enhanced deserialize_flyte_file validator to check if private attributes exist
   • If missing, reconstructs the FlyteFile using dict_to_flyte_file() transformer
   • If attributes already exist (e.g., when passing already-constructed FlyteFile), returns as-is

2. FlyteDirectory (flytekit/types/directory/types.py):

   • Applied same fix to deserialize_flyte_dir validator
   • Uses dict_to_flyte_directory() to properly reconstruct the object

How was this patch tested?

Added two new unit tests in test_pydantic_basemodel_transformer.py:

1. test_flytefile_pydantic_model_dump_validate_cycle - Verifies FlyteFile can be serialized, deserialized, and re-serialized without errors
2. test_flytedirectory_pydantic_model_dump_validate_cycle - Same for FlyteDirectory

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

Summary by Bito

This pull request fixes a critical bug in deserializing Pydantic models with FlyteFile and FlyteDirectory fields by adding model validators. These validators ensure proper initialization of private attributes, preventing AttributeErrors during re-serialization. New unit tests have also been included to confirm the functionality of these enhancements.

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