feat: introduce IncompatibleTransaction error

[wjones127]

From #5983 (comment), we currently use CommitConflict for two situations:

1. Incompatible transactions: there is a conflict that is not retry-able. For example, you are trying to create an index, but a concurrent transaction overwrote the table and changed the schema.
2. Commit step ran out of retries: we hit the max number of rebase attempts, and even though we could retry again, we aren't. This is indeed just throttling.

This makes them separate errors.

[github-actions]

Code Review: PR #6003 - feat: introduce IncompatibleTransaction error

Summary

This PR introduces a new IncompatibleTransaction error type to distinguish between non-retryable transaction conflicts and conflicts that exhausted retries. The semantic separation is correct and improves error clarity.

P1 Issue: Java bindings inconsistency

The Java error mapping at java/lance-jni/src/error.rs:207 explicitly handles CommitConflict → IllegalArgumentException, but the new IncompatibleTransaction error will fall through to the default case (RuntimeException):

LanceError::DatasetNotFound { .. }
| LanceError::DatasetAlreadyExists { .. }
| LanceError::CommitConflict { .. }  // ← explicitly handled
| LanceError::InvalidInput { .. } => Self::input_error(err.to_string()),
// IncompatibleTransaction falls through to RuntimeException

Since IncompatibleTransaction represents a semantic transaction conflict (arguably more severe than CommitConflict), it should likely be mapped consistently. Consider adding LanceError::IncompatibleTransaction { .. } to the same match arm, or make a deliberate choice about how Java clients should handle this new error type.

---

The rest of the changes look correct - the error variant is well-defined, tests have been updated appropriately, and the error message provides good context about the incompatibility.

[codecov]

Codecov Report

❌ Patch coverage is 38.09524% with 13 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/io/commit/conflict_resolver.rs	38.46%	0 Missing and 8 partials ⚠️
rust/lance/src/index/mem_wal.rs	50.00%	0 Missing and 3 partials ⚠️
rust/lance/src/io/commit.rs	0.00%	0 Missing and 2 partials ⚠️

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