fix: branch_identfier unstable for legacy branches

[majin1102]

Problem

Legacy branches, i.e. branches whose BranchContents were written without a persisted branch_identifier, currently deserialize through BranchIdentifier::none(). That fallback generates a fresh random UUID on each read, so the same unchanged branch can surface a different branch_identifier across repeated loads.

This makes branch identity unstable in both Python and Java for legacy datasets. On the Python side, branches.list() / branches_ordered() expose branch_identifier directly, so callers that diff, cache, or snapshot branch metadata can observe false changes even when the branch itself has not changed. On the Java side, the same legacy branch can also appear with a different identifier across refreshes, which makes equality-style comparisons unstable as well.

Summary

• stabilize fallback branch identifiers for legacy branch metadata by replacing the missing-identifier sentinel with a deterministic synthetic UUID during branch metadata reads
• keep the fallback logic localized to Rust branch metadata loading so Python and Java continues returning stable branch_identifier values without API shape changes
• add a lightweight Rust regression test that exercises BranchContents::from_path on in-memory branch metadata and verifies stable repeated reads plus distinct identifiers for different branch names

[codecov]

Codecov Report

❌ Patch coverage is 97.18310% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/dataset/refs.rs	97.18%	0 Missing and 2 partials ⚠️

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[majin1102]

@codex review

[chatgpt-codex-connector]

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