fix: add regression tests for checksums

[ferhatelmas]

What kind of change does this PR introduce?

Tests

What is the current behavior?

No coverage for fixes in #984

What is the new behavior?

Some coverage to prevent regression.

Additional context

#982

[Copilot]

Pull request overview

Adds targeted regression tests to prevent reintroducing previously fixed S3 multipart checksum mapping and missing-parameter reporting issues (from #984 / #982), improving confidence in the S3 protocol/router behavior.

Changes:

• Adds a test verifying the Iceberg CompleteMultipartUpload route maps ChecksumCRC32C from the backend response to the correct response field.
• Adds a test verifying S3ProtocolHandler.completeMultiPartUpload() preserves ChecksumCRC32C in its response mapping.
• Adds tests ensuring headObject and dbHeadObject report Key (not Bucket) as the missing parameter when Key is absent.

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[coveralls]

Coverage Report for CI Build 24071766404

Coverage increased (+0.1%) to 80.664%

Details

• Coverage increased (+0.1%) from the base build.
• Patch coverage: No coverable lines changed in this PR.
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	37229
Covered Lines:	30214
Line Coverage:	81.16%
Relevant Branches:	4113
Covered Branches:	3134
Branch Coverage:	76.2%
Branches in Coverage %:	Yes
Coverage Strength:	314.52 hits per line

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💛 - Coveralls