Refine FileLike Protocol to be compatible with pandas ReadPickleBuffer

[hiro-o918]

Background

In #517, pd.read_pickle(file) requires cast(Any, file) to suppress a type error because the current FileLike Protocol defined in gokart/utils.py is not structurally compatible with pandas' ReadPickleBuffer Protocol.

# gokart/utils.py
return pd.read_pickle(cast(Any, file))

Current FileLike

class FileLike(Protocol):
    def read(self, n: int) -> bytes: ...
    def readline(self) -> bytes: ...
    def seek(self, offset: int) -> None: ...
    def seekable(self) -> bool: ...

pandas ReadPickleBuffer requirements

mode: str                                       # property
read(n: int = ..., /) -> bytes
readline() -> bytes
seek(offset, whence=..., /) -> int
seekable() -> bool
tell() -> int

Proposal

Refine FileLike so that it is a structural subtype of ReadPickleBuffer. This would let us drop the cast(Any, file) workaround.

Required changes:

1. Update FileLike signatures in gokart/utils.py:
   • Add mode property
   • read(n: int = ..., /) -> bytes (default + positional-only)
   • seek(offset: int, whence: int = ..., /) -> int (return int, accept whence)
   • Add tell() -> int
2. Update _ChunkedLargeFileReader in gokart/file_processor/base.py to match the new signatures and explicitly expose mode / tell (currently they only work at runtime via __getattr__).
3. Simplify FileLike | BytesIO to just FileLike since BytesIO already satisfies ReadPickleBuffer.

Notes

• Tracked separately from add casts for pandas type stub incompatibilities #517 to keep that PR minimal.
• Need to confirm all real callers (BytesIO, _ChunkedLargeFileReader wrapping S3/GCS streams) actually expose mode / tell.