feat(storage): Enable full object checksum PR 1/3 : parse finalize_time and server crc32c in async object stream

[chandra-siri]

1. Overview of the Solution

This solution implements end-to-end full-object checksum validation in AsyncMultiRangeDownloader for the asynchronous Google Cloud Storage Python client library. As asynchronous multiplexed downloads of non-contiguous ranges are performed concurrently over a single bidirectional gRPC connection, this feature automatically and incrementally calculates a rolling checksum as bytes arrive and validates it against the server's authoritative object checksum once the download completes.

The technical approach consists of three coordinated layers:

• _AsyncReadObjectStream (Stream Ingestion): Safely extracts the authoritative server checksum (full_obj_server_crc32c) and finalization status (is_finalized) from the object metadata received in the first data payload response of the stream.
• _ReadResumptionStrategy & _DownloadState (Verification Logic): Computes an isolated, persistent rolling checksum in the individual _DownloadState object to ensure calculations do not bleed across concurrent multiplexed ranges. Crucially, the rolling hash updates only after buffer writes succeed to prevent state corruption during retry re-connects, raising a DataCorruption exception on completion if a mismatch occurs.
• AsyncMultiRangeDownloader (Orchestration & Cleanup): Detects candidate full-object ranges (e.g., (0, 0) or (0, persisted_size)), propagates checksum settings to the resumption strategy, and guarantees robust cleanup (closing the stream immediately and unregistering IDs) if data corruption or write errors occur.

2. What This PR Specifically Does

This PR implements Step 1: Stream Metadata Ingestion of the solution:

• Modifies _AsyncReadObjectStream to safely parse GCS object metadata from the first data payload of the response.
• Populates is_finalized, full_obj_server_crc32c, and object_metadata attributes in _AsyncReadObjectStream.open().
• Adds an autouse pytest event loop fixture in tests/unit/conftest.py to resolve compatibility issues with pytest-asyncio under Python 3.11+.
• Adds unit tests in test_async_read_object_stream.py to verify that finalization status and server-authoritative checksums are correctly extracted or skipped for unfinalized objects.

[gemini-code-assist]

Code Review

This pull request introduces tracking for object finalization and full object CRC32C checksums in the asynchronous read stream. The feedback focuses on simplifying the production code by removing logic added solely to accommodate unit test mocks (such as checking for a .seconds attribute). Instead, it is recommended to mock finalize_time as a standard datetime.datetime or None in the unit tests, which allows the production code to rely on standard isinstance checks.