fix(files): check is_tuple_t before is_file_content to fix PathLike inside file tuples

[IgnazioDS]

Summary

Fixes #1318 — client.beta.files.upload crashes when a PathLike is passed as the second element of a file tuple (e.g. ("filename.txt", Path("foo.txt"), "text/plain")).

Root cause: is_file_content() returns True for any tuple, so the is_tuple_t branch in both _transform_file and _async_transform_file was unreachable. A file tuple hit the is_file_content branch first and was returned unchanged — the PathLike inside was never read via read_file_content.

# Before — is_file_content catches tuples, is_tuple_t is unreachable
def _transform_file(file: FileTypes) -> HttpxFileTypes:
    if is_file_content(file):   # True for tuples → returns tuple as-is
        ...
        return file              # PathLike inside tuple never read

    if is_tuple_t(file):        # UNREACHABLE for tuples
        return (file[0], read_file_content(file[1]), *file[2:])

Fix: Check is_tuple_t first in both the sync and async paths, so tuples with PathLike second elements are correctly read before being forwarded to httpx.

# After — tuple branch runs first
def _transform_file(file: FileTypes) -> HttpxFileTypes:
    if is_tuple_t(file):
        return (file[0], read_file_content(file[1]), *file[2:])  # PathLike read ✓

    if is_file_content(file):
        if isinstance(file, os.PathLike):
            ...
        return file

Same fix applied to _async_transform_file with async_read_file_content.

Test plan

• ("filename.txt", Path("foo.txt"), "text/plain") — PathLike read, no crash
• ("filename.txt", Path("foo.txt")) — 2-tuple variant works
• Path("foo.txt") bare PathLike — still works (goes through is_file_content branch)
• ("filename.txt", b"bytes", "text/plain") — bytes tuple still works
• Run pytest tests/test_files.py