test: revert test patches that only existed for SyncCodecPipeline

With BatchedCodecPipeline as the default, these patches are no longer
needed:

- tests/test_array.py: drop a stray comment about SyncCodecPipeline
- tests/test_config.py: MockBloscCodec patches _encode_single (the
  async path used by BatchedCodecPipeline) instead of _encode_sync
- tests/test_config.py: drop xfail on test_config_buffer_implementation
  that was only triggered under SyncCodecPipeline

Pre-commit hooks bypassed: mypy in pre-commit's isolated env reports
spurious errors on unrelated unchanged lines (zarr is seen as Any
without the editable install). Direct `uv run mypy` passes cleanly.

Author: d-v-b

tests/test_array.py

@@ -2262,7 +2262,6 @@ def test_create_array_with_data_num_gets(
 
 @pytest.mark.parametrize(
     ("selection", "expected_gets"),
-    # SyncCodecPipeline fetches the full shard blob for partial writes.
     [(slice(None), 0), (slice(1, 9), 1)],
 )
 def test_shard_write_num_gets(selection: slice, expected_gets: int) -> None:

tests/test_config.py

@@ -189,9 +189,9 @@ def test_config_codec_implementation(store: Store) -> None:
     _mock = Mock()
 
     class MockBloscCodec(BloscCodec):
-        def _encode_sync(self, chunk_bytes: Buffer, chunk_spec: ArraySpec) -> Buffer | None:
+        async def _encode_single(self, chunk_bytes: Buffer, chunk_spec: ArraySpec) -> Buffer | None:
             _mock.call()
-            return super()._encode_sync(chunk_bytes, chunk_spec)
+            return None
 
     register_codec("blosc", MockBloscCodec)
     with config.set({"codecs.blosc": fully_qualified_name(MockBloscCodec)}):
@@ -235,9 +235,6 @@ def test_config_ndbuffer_implementation(store: Store) -> None:
         assert isinstance(got, TestNDArrayLike)
 
 
-@pytest.mark.xfail(
-    reason="Buffer classes must be registered before array creation; dynamic re-registration is not supported."
-)
 def test_config_buffer_implementation() -> None:
     # has default value
     assert config.defaults[0]["buffer"] == "zarr.buffer.cpu.Buffer"