test(storage): update test_mrd_open_with_read_handle_over_cloud_path for CloudPath read_handle support - #19051

Summary

Updates test_mrd_open_with_read_handle_over_cloud_path in packages/google-cloud-storage/tests/system/test_rapid.py to assert assert new_mrd.persisted_size is None (matching test_mrd_open_with_read_handle over DirectPath).

Why This Is Needed

1. How persisted_size Is Populated:
   In _AsyncReadObjectStream.open() (packages/google-cloud-storage/google/cloud/storage/asyncio/async_read_object_stream.py), self.persisted_size = response.metadata.size is only populated when the server returns response.metadata (Object proto) in the initial BidiReadObjectResponse.
2. Behavior When Opening with read_handle:
   When a BidiReadObject stream is opened using an existing read_handle (BidiReadObjectSpec.read_handle), the server fast-path skips metadata lookup and omits response.metadata from the initial response. Consequently, new_mrd.persisted_size remains None (as verified in test_mrd_open_with_read_handle over DirectPath).
3. GCS Backend Support for read_handle over CloudPath:
   Previously, test_mrd_open_with_read_handle_over_cloud_path asserted assert new_mrd.persisted_size == len(_BYTES_TO_UPLOAD) because the GCS backend did not support read_handle over CLOUD_PATH (attempt_direct_path=False), falling back to a full open that returned response.metadata.
   The GCS backend now honors read_handle over CLOUD_PATH just like DIRECT_PATH, omitting response.metadata on initial open so new_mrd.persisted_size is None.

await new_mrd.open()
# persisted_size not set when opened with read_handle
assert new_mrd.persisted_size is None

[Generated-by: AI]