fix(files): resolve PathLike inside FileTypes tuples

[qozle]

What

Fixes #1318.

is_file_content() in _files.py included isinstance(obj, tuple) in its check, but FileContent = Union[IO[bytes], bytes, PathLike[str]] — tuples are not file content, they are FileTypes variants (the (name, content, content_type[, headers]) forms).

This meant the is_tuple_t(file) branch in _transform_file and _async_transform_file was unreachable for any tuple input. Instead of calling read_file_content(file[1]) on the inner element, both functions returned the tuple unchanged. When that inner element was a PathLike, httpx received it as-is and failed.

Why it worked before (sort of)

Passing ("name", b"bytes") as a FileTypes value still produced a usable result because httpx accepts raw tuples with bytes. But ("name", Path("foo"), "text/plain") failed because httpx does not read PathLike objects — and the type annotation in _types.py explicitly promises PathLike is a valid FileContent.

Fix

Remove isinstance(obj, tuple) from is_file_content. Tuples now fall through to the is_tuple_t branch and get processed correctly — read_file_content / async_read_file_content resolves the PathLike to bytes before passing to httpx.

Also corrects the assert_is_file_content error message, which listed "or a tuple" as a valid FileContent form.

Tests

Added test_pathlike_in_tuple and test_async_pathlike_in_tuple which exercise the previously broken path. All existing tests pass.

# was silently broken
to_httpx_files({"file": ("report.pdf", Path("report.pdf"), "application/pdf")})
# now works correctly