[PY] Support pickling FFI String and Bytes

[Ubospica]

Summary

• Add pickle reductions for tvm_ffi.core.String and tvm_ffi.core.Bytes so only the Python payload is serialized.
• Ensure unpickled values drop the hidden _tvm_ffi_cached_object backing object.
• Add regression coverage for cached FFI String/Bytes returned from testing.echo.

Notes

• This is a clean branch based on the latest apache/main and supersedes [PY] Support pickling ffi.String without cached FFI object #642, which became dirty after main advanced.
• Bytes.__slots__ is intentionally unchanged because Python does not support non-empty __slots__ on bytes subclasses.

Test plan

• uv pip install --reinstall --verbose . && .venv/bin/pytest -vvs tests/python/test_string.py
• PYTHONPATH=/raid/user_data/yixind/miniforge3/lib/python3.12/site-packages pre-commit run --files python/tvm_ffi/cython/string.pxi tests/python/test_string.py

[gemini-code-assist]

Code Review

This pull request implements custom pickling support for the String and Bytes classes in python/tvm_ffi/cython/string.pxi by adding __reduce_ex__ methods, along with corresponding unit tests to verify that cached FFI objects are cleared during roundtrip serialization. The reviewer feedback points out that the current __reduce_ex__ implementations return a 2-tuple, which discards the instance's __dict__ and causes any custom or subclass attributes to be silently lost during pickling. It is recommended to return the state dictionary (excluding _tvm_ffi_cached_object) as the third element of the reduction tuple to preserve these attributes.

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[Ubospica]

Addressed the latest Gemini comments in cb9272a:

• String.__reduce_ex__ and Bytes.__reduce_ex__ now return state as the third reduction element.
• The state keeps ordinary instance attributes and drops only _tvm_ffi_cached_object.
• Added tests for String subclass state and Bytes instance state preservation.

[Ubospica]

Updated the pickle semantics in a030c1f per requested behavior:

• pickle.loads(pickle.dumps(tvm_ffi.core.String(...))) now returns raw Python str.
• pickle.loads(pickle.dumps(tvm_ffi.core.Bytes(...))) now returns raw Python bytes.
• Tests now assert exact native types for both directly constructed values and cached FFI-returned values.

This intentionally drops FFI wrapper type/subclass state across pickle, matching the desired raw Python payload behavior.