fix: avoid panic when rebasing a commit whose read version was cleaned up

[qiuyuhang]

Problem

When two transactions commit concurrently and one of them needs to be rebased,
initial_fragments_for_rebase calls checkout_version(transaction.read_version)
and .unwrap()s the result:

dataset
    .checkout_version(transaction.read_version)
    .await
    .unwrap(),

If a concurrent cleanup_old_versions (e.g. triggered by VACUUM) removes that
version's manifest between the conflicting commit and the rebase, checkout_version
returns DatasetNotFound and the unwrap() panics. In builds compiled with
panic = "abort" (common when Lance is embedded via FFI) this aborts the entire
host process instead of failing just the commit.

Observed in production as:

thread '<unnamed>' panicked at .../lance/src/io/commit/conflict_resolver.rs:
called `Result::unwrap()` on an `Err` value: DatasetNotFound {
    path: ".../_versions/85.manifest", ...
}

Fix

Return a Result from initial_fragments_for_rebase and propagate the error
instead of unwrapping. A commit whose read version has been garbage-collected now
fails gracefully with DatasetNotFound, allowing the caller to retry, rather than
panicking. All 5 call sites are updated to use ?.

This is a behavior change only for the previously-panicking path; the success path
is unchanged.

Test

Added test_rebase_errors_when_read_version_was_cleaned_up, which:

1. writes two versions of a dataset,
2. builds a transaction pinned to version 1,
3. deletes version 1's manifest to simulate concurrent cleanup,
4. asserts TransactionRebase::try_new returns DatasetNotFound instead of panicking.

Verified the test fails (panics) without the fix and passes with it.

Verification

• cargo check -p lance — clean
• cargo test -p lance --lib io::commit::conflict_resolver — 59 passed (58 existing + 1 new)

[lance-gatekeeper]

✅ Gate recommendation: approve.

The rebase now propagates the existing DatasetNotFound contract from the authoritative historical checkout instead of panicking, with no durable write performed before the failure. The regression coverage exercises the cleaned-up-manifest path while preserving the surrounding conflict-resolution behavior.

[zhangyue19921010]

LGTM. Thanks for your contribution!