feat(storage): full object checksum: implement rolling checksum and verification in reads resumption strategy

[chandra-siri]

PR 2 of 3: Upgrade _DownloadState to support rolling checksums, and verify full-object crc32c at completion in _ReadResumptionStrategy.

[gemini-code-assist]

Code Review

This pull request introduces support for full-object rolling checksum verification during resumed reads, allowing verification once the stream completes. It also adds a flag to conditionally disable checksum validation. The review feedback suggests optimizing performance by bypassing rolling checksum updates when checksums are disabled, and recommends adding a unit test to verify that full-object checksum mismatches are ignored when checksum validation is disabled.

[gemini-code-assist]

If checksum_enabled is False, we should avoid updating the rolling checksum to prevent unnecessary CPU overhead, especially when downloading large objects where checksums have been explicitly disabled for performance.

Suggested change

	if read_state.rolling_checksum is not None:
	read_state.rolling_checksum.update(data)
	if checksum_enabled and read_state.rolling_checksum is not None:
	read_state.rolling_checksum.update(data)

[gemini-code-assist]

To ensure comprehensive test coverage, we should also verify that the full-object checksum verification is bypassed when enable_checksum is set to False.

    def test_update_state_checksum_mismatch_ignored_when_disabled(self):
        """Test that a CRC32C mismatch is ignored when enable_checksum is False."""
        self._add_download(_READ_ID)
        self.state["enable_checksum"] = False
        response = self._create_response(b"data", _READ_ID, offset=0, crc=999999)

        # Should NOT raise DataCorruption!
        self.strategy.update_state_from_response(response, self.state)

    def test_update_state_full_object_checksum_ignored_when_disabled(self):
        """Test that full object checksum mismatch is ignored when enable_checksum is False."""
        self._add_download(_READ_ID, offset=0, length=9, is_full_object_read=True)
        self.state["enable_checksum"] = False
        self.state["full_obj_server_crc32c"] = 111111  # Wrong server checksum!

        resp1 = self._create_response(b"test", _READ_ID, offset=0)
        self.strategy.update_state_from_response(resp1, self.state)

        resp2 = self._create_response(b"data1", _READ_ID, offset=4, range_end=True)
        # Should NOT raise DataCorruption!
        self.strategy.update_state_from_response(resp2, self.state)