feat(storage): expose object_metadata on AsyncMultiRangeDownloader

[yuxin00j]

This PR exposes the object_metadata attribute on the AsyncMultiRangeDownloader class.

Context

In PR #17261, object_metadata was successfully exposed on the AsyncReadObjectStream. However, it was not surfaced on the AsyncMultiRangeDownloader (MRD), meaning high-level libraries relying on MRD (like fsspec/gcsfs) cannot access the underlying gRPC metadata.

By inheriting and surfacing self.object_metadata = self.read_obj_str.object_metadata, this allows downstream users to completely bypass redundant RESTinfo() API calls and extract fields like crc32c, size, and generation directly from the MRD instance.

cc @chandra-siri

[gemini-code-assist]

Code Review

This pull request adds tracking for object metadata in the AsyncMultiRangeDownloader class by initializing and copying object_metadata from the underlying stream. The reviewer suggests refactoring this attribute into a read-only property that dynamically delegates to the stream, which simplifies the code by removing the need for manual assignments in multiple places.

[chandra-siri]

Hi @yuxin00j

Can you also add a system test for the same in packages/google-cloud-storage/tests/system/test_zonal.py

since this PR is approved, you may do it in follow up PR