fix: make overwrites retryable instead of compatible

[jackye1995]

From a Snapshot isolation perspective, 2 overwrites should not just be compatible because they have modified overlapping data between 2 writes. We will mark it as retryable and client can make a decision if it can be retried or not depending on the specific case.

[github-actions]

Code Review

Summary: This PR correctly changes overwrite conflict semantics for better Snapshot isolation: two concurrent creates (read_version == 0) are now incompatible, while overwrites on existing datasets return a retryable conflict for client decision-making.

Review Result: ✅ Looks good to merge

The logic is sound:

• Two creates must be incompatible since only one can create the dataset
• Overwrites on existing data should be retryable so clients can decide

Test coverage is adequate:

• Test matrix updated to reflect create vs create behavior
• test_concurrent_overwrites_retryable validates retryable semantics for existing datasets
• test_concurrent_create_incompatible validates incompatible semantics for creates

No P0/P1 issues identified.

[codecov]

Codecov Report

❌ Patch coverage is 93.06931% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/io/commit/conflict_resolver.rs	90.16%	5 Missing and 1 partial ⚠️
rust/lance/src/io/commit.rs	97.50%	0 Missing and 1 partial ⚠️

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