GitBlobstore: implement CheckAndPut CAS semantics + add tests

[coffeegoddd]

This PR adds the next write-path primitive to GitBlobstore: CheckAndPut with proper compare-and-swap behavior, and a focused test suite (including a concurrency/CAS-failure scenario).

[Copilot]

Pull request overview

This PR implements the CheckAndPut method for GitBlobstore with proper compare-and-swap (CAS) semantics. The implementation validates the expected version before consuming the reader, creates a commit with the new blob, and uses git's UpdateRefCAS to atomically update the reference. The PR includes comprehensive test coverage for create-only operations, version mismatches, successful updates, and concurrent update scenarios.

Changes:

• Implemented CheckAndPut with CAS semantics that validates version before consuming the reader and handles both create and update operations
• Refactored buildPutCommit to extract buildCommitWithMessage for code reuse between Put and CheckAndPut
• Added comprehensive test suite including tests for create-only, version mismatch, successful update, and concurrent update scenarios

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
go/store/blobstore/git_blobstore.go	Implements CheckAndPut method with CAS semantics and refactors commit building logic into buildCommitWithMessage for reuse
go/store/blobstore/git_blobstore_test.go	Adds failReader helper and four comprehensive tests covering create-only, version mismatch, successful update, and concurrent update scenarios

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[coffeegoddd]

@coffeegoddd DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
a93d875	ok	5937471

version	total_tests
a93d875	5937471

correctness_percentage
100.0

[reltuk]

LGTM!