add HashObject + GitAPIImpl write-primitive tests

[coffeegoddd]

This PR extends the unified internal/git plumbing API with a streaming blob-write primitive and adds targeted unit coverage for the write building blocks we’ll use to implement GitBlobstore write paths.

[Copilot]

Pull request overview

This PR adds a new HashObject method to the internal Git plumbing API that enables streaming blob writes, and includes comprehensive unit tests for the write-path building blocks. The implementation wraps the git hash-object -w --stdin command to create blob objects directly in the Git object database.

Changes:

• Added HashObject method to the GitAPI interface for writing blob objects from streaming content
• Implemented HashObject in GitAPIImpl using the git CLI plumbing command
• Added 7 comprehensive test cases covering blob writing, tree manipulation, ref updates, and commit operations

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
go/store/blobstore/internal/git/api.go	Added HashObject interface method definition with documentation
go/store/blobstore/internal/git/impl.go	Implemented HashObject method with output parsing and error handling
go/store/blobstore/internal/git/impl_test.go	Added comprehensive test coverage for HashObject and other write primitives including round-trip tests, empty blob handling, tree operations, ref management, and CAS updates

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[coffeegoddd]

@coffeegoddd DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
d25cbfe	ok	5937471

version	total_tests
d25cbfe	5937471

correctness_percentage
100.0