Move the registry tests into test_s3.py

Keep the test file layout mirroring the source layout instead of adding
a separate test_registry.py.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: laughingman7743

tests/pyathena/filesystem/test_registry.py

@@ -1,4 +1,5 @@
 import io
+import logging
 import os
 import tempfile
 import time
@@ -12,8 +13,10 @@
 from types import SimpleNamespace
 from unittest import mock
 
+import fsspec
 import pytest
 from fsspec import Callback
+from fsspec.registry import _registry, known_implementations
 
 from pyathena.filesystem import register_s3_filesystem
 from pyathena.filesystem.s3 import S3File, S3FileSystem
@@ -1653,3 +1656,43 @@ def test_upload_chunk_multipart(self, autocommit):
             file.commit()
         file.fs._finish_multipart_upload.assert_called_once()
         file.fs._put_object.assert_not_called()
+
+
+class _DummyFileSystem:
+    pass
+
+
+@pytest.fixture
+def registry_state():
+    registry = dict(_registry)
+    known = {k: dict(v) for k, v in known_implementations.items()}
+    yield
+    _registry.clear()
+    _registry.update(registry)
+    known_implementations.clear()
+    known_implementations.update(known)
+
+
+def test_register_s3_filesystem(registry_state):
+    _registry.pop("s3", None)
+    _registry.pop("s3a", None)
+
+    register_s3_filesystem()
+    assert fsspec.registry["s3"] is S3FileSystem
+    assert fsspec.registry["s3a"] is S3FileSystem
+
+
+def test_register_s3_filesystem_overwrites_existing_registration(registry_state, caplog):
+    fsspec.register_implementation("s3", _DummyFileSystem, clobber=True)
+
+    with caplog.at_level(logging.WARNING, logger="pyathena.filesystem"):
+        register_s3_filesystem()
+    # An explicitly registered filesystem class is overwritten with a warning.
+    assert fsspec.registry["s3"] is S3FileSystem
+    assert "will be overwritten" in caplog.text
+
+    # Re-registering PyAthena's own filesystem does not warn.
+    caplog.clear()
+    with caplog.at_level(logging.WARNING, logger="pyathena.filesystem"):
+        register_s3_filesystem()
+    assert not caplog.text