feat(storage): full object checksum: integrate full-object checksum in AsyncMultiRangeDownloader

[chandra-siri]

PR 3 of 3: Connect full-object checksum in AsyncMultiRangeDownloader, close stream on corruption, and add end-to-end system tests.

[gemini-code-assist]

Code Review

This pull request introduces checksum validation for full object downloads in the AsyncMultiRangeDownloader by tracking rolling checksums and handling DataCorruption exceptions. It also defers the CRC32C C-extension check to the download phase. The review feedback points out an optimization opportunity in the is_full_object_read heuristic to prevent unnecessary CRC32C computations when checksums are disabled or when the object is unfinalized.

[gemini-code-assist]

The current heuristic for is_full_object_read does not take into account whether checksum validation is enabled (enable_checksum) or if the object is finalized (self.is_finalized).

1. If enable_checksum is False: The downloader will still set is_full_object_read to True and initialize rolling_checksum in _DownloadState, incurring unnecessary CRC32C computation overhead on every chunk downloaded.
2. If self.is_finalized is False: For unfinalized appendable objects, there is no finalized server-side CRC32C to validate against. Computing the rolling checksum in this case is a waste of CPU cycles.

We should restrict is_full_object_read to only be True when both enable_checksum and self.is_finalized are True.

            is_full_object_read = (
                enable_checksum and
                self.is_finalized and
                (
                    (offset == 0 and length == 0) or
                    (self.persisted_size is not None and offset == 0 and length == self.persisted_size)
                )
            )