fix(storage): attach trailing checksum on final chunk of streaming resumable uploads

[chandra-siri]

Fixes b/471191823.

Root Cause

During streaming resumable uploads where initial content length is unknown (total_bytes starts as None), the SDK previously evaluated (start_byte + len(payload) == self._total_bytes) to determine when to append the trailing x-goog-hash checksum header. Because self._total_bytes remained None, this condition evaluated to False on the terminal chunk, causing the checksum header to be omitted from the final PUT request.

Fix

In ResumableUpload._prepare_request(), when get_next_chunk() detects EOF on an unknown-length stream (indicated by content_range transitioning from bytes start-end/* to bytes start-end/total), we now update self._total_bytes to the parsed total byte size. Consequently, the terminal chunk request properly attaches x-goog-hash.

Verification

• Added positive and negative unit tests verifying x-goog-hash attachment on streaming uploads.
• Added live GCS system tests validating trailing CRC32C verification and rejection on corrupted stream checksums.

[gemini-code-assist]

Code Review

This pull request adds support for extracting and setting the total bytes during a resumable streaming upload when the size is initially unknown, enabling trailing checksum validation. It also includes comprehensive unit and system tests to verify this behavior. The review feedback suggests a cleaner and more robust approach to update self._total_bytes by directly using the total_bytes local variable instead of parsing the content_range string.