fix(version): preserve unspecified commit placeholder

[stephenlclarke]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

Supersedes #1831, whose deleted fork head cannot be reattached to the closed review.

When container is built without a GIT_COMMIT value, the C version shim returns unspecified. ReleaseVersion.singleLine(appName:) currently shortens every non-nil commit string to seven characters, so the version banner reports commit: unspeci.

The change preserves missing, empty, and unspecified commit values as the full placeholder while retaining the seven-character display for real commit hashes.

What Changed

• Centralizes commit display formatting in ReleaseVersion.displayCommit(_:).
• Preserves the complete unspecified placeholder.
• Keeps seven-character short hashes for real commits.
• Adds focused tests for nil, empty, placeholder, and real-hash values.

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

Validation:

• make swift-fmt-check
• make test: 94 XCTest cases and 560 Swift Testing cases passed
• swift test --disable-automatic-resolution --enable-code-coverage --filter ReleaseVersionTests
• env -u GIT_COMMIT swift build --disable-automatic-resolution --product container
• env -u GIT_COMMIT .build/debug/container --version
• git diff --check origin/main..HEAD

The version smoke check reports:

container CLI version 0.0.0 (build: debug, commit: unspecified)

The coverage run executes every branch and return path added by displayCommit(_:).