fix(files): remove erroneous tuple check from is_file_content type guard (#1318)

[xodn348]

Summary

Fixes #1318.

client.beta.files.upload(file=("foo.txt", Path("foo.txt"), "text/plain")) fails because the is_file_content type guard incorrectly matches tuples, preventing the tuple-specific branch in _transform_file from handling PathLike resolution.

Problem

is_file_content() included isinstance(obj, tuple) despite FileContent being defined as Union[IO[bytes], bytes, PathLike] (no tuple). This caused _transform_file to enter the FileContent branch for tuple inputs, skipping the is_tuple_t branch where read_file_content properly resolves PathLike elements.

Fix

Remove the erroneous isinstance(obj, tuple) check from is_file_content, aligning the runtime type guard with the static FileContent type definition. Tuples now correctly reach the is_tuple_t branch and have their PathLike elements opened before being passed to httpx.

Change

 def is_file_content(obj: object) -> TypeGuard[FileContent]:
-    return (
-        isinstance(obj, bytes) or isinstance(obj, tuple) or isinstance(obj, io.IOBase) or isinstance(obj, os.PathLike)
-    )
+    return isinstance(obj, bytes) or isinstance(obj, io.IOBase) or isinstance(obj, os.PathLike)