[PY] Support pickling ffi.String without cached FFI object

[Ubospica]

Summary

• Add a custom pickle reduction for tvm_ffi.core.String so only the Python string payload is serialized.
• Drop the hidden _tvm_ffi_cached_object on unpickle, since String is effectively str with optional FFI backing.
• Add a regression test for long FFI strings that carry a cached backing object.

[gemini-code-assist]

Code Review

This pull request implements __reduce_ex__ for the String class in Cython to prevent the serialization of the cached FFI object during pickling, and adds corresponding unit tests to verify this behavior. The reviewer suggests extending this fix to the Bytes class, which shares the same caching behavior, by implementing __reduce_ex__ and adding a corresponding test case.

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[Ubospica]

Addressed the Gemini comments in db208dc:

• Added Bytes.__reduce_ex__ so pickling tvm_ffi.core.Bytes only persists the Python bytes payload and drops _tvm_ffi_cached_object on unpickle.
• Added a regression test for cached FFI Bytes returned by testing.echo.
• Kept Bytes.__slots__ unchanged because Python raises TypeError: nonempty __slots__ not supported for subtype of 'bytes'; the reduction hook is sufficient to avoid serializing the cached FFI object.

Local validation:

• uv pip install --reinstall --verbose . && .venv/bin/pytest -vvs tests/python/test_string.py
• commit hooks passed after fixing the local pre-commit cache dependency versions for mypy (numpy<2.0, pytest<8) to match the repo's Python 3.9 mypy target.

[Ubospica]

Superseded by #643, which is based on the latest and includes the Gemini Bytes review fixes without the dirty merge state.

[Ubospica]

Closing in favor of #643.