fix: set _row_created_at_version to new version for MERGE INTO INSERT rows

[jerryjch]

Summary

• Fixes Newly inserted rows in a fragment-rewrite commit (e.g. MERGE INTO) get _row_created_at_version from a prior version instead of the commit version #6735.
• transaction.rs — resolve_update_version_metadata: in the per-row created_at
  mapping, check row_id_to_source.contains_key(&rid) before calling
  resolve_created_at_version. Rows not in the map are INSERT branch rows (no source
  in existing fragments); they now receive new_version as created_at instead of the
  previous fallback of UNKNOWN_CREATED_AT_VERSION (1).
• resolve_created_at_version doc comment updated to clarify it is only called for
  UPDATE branch rows (source confirmed present). The unmapped-row-ID branch inside the
  function is unused when called from resolve_update_version_metadata; UNKNOWN (1)
  still applies for UPDATE rows whose source fragment has missing or bad
  created_at_version_meta (cache miss, decode failure, or out-of-range offset).
• Two existing tests updated to assert the corrected behavior; one new test added.

Background

MERGE INTO commits through Operation::Update and produces both UPDATE branch rows
(rewritten into new_fragments with a source row in the previous manifest, stable row
ID present in row_id_to_source) and INSERT branch rows (new rows also in
new_fragments, stable row ID assigned fresh, not present in any existing fragment).

Before this change, resolve_update_version_metadata built the per-row
created_at_versions vector by calling resolve_created_at_version for every row ID.
For UPDATE branch rows that function correctly copies created_at from the source
fragment. For INSERT branch rows the map lookup fails and the function returns
UNKNOWN_CREATED_AT_VERSION = 1, producing a wrong historical version for every newly
inserted row. CDF consumers cannot distinguish merge-inserted rows from updated rows via
_row_created_at_version, and the value 1 is meaningless for rows that first appeared
in a recent commit.

The fix is a single guard at the call site: only call resolve_created_at_version for
rows confirmed to have a source (UPDATE branch); for all other rows use new_version
directly.

Implementation notes

• The guard uses row_id_to_source.contains_key(&rid), which is an O(1) hash lookup
  on the same map already built for the UPDATE branch path — no additional data
  structures or iteration.
• No lance-spark changes are needed. The Spark commit path (SparkPositionDeltaWrite)
  already attaches RowIdMeta to new fragment rows. This change activates the correct
  behavior automatically for all callers of Operation::Update, including lance-spark
  MERGE INTO.
• lane-spark test update test: update MergeIntoTest lance-spark#530

Test plan

• test_update_version_tracking_insert_branch_gets_new_version (renamed from
  test_update_version_tracking_unknown_row_id_defaults_to_1): new fragment with one
  UPDATE branch row (ID 10, source created_at = 5) and one INSERT branch row (ID 999);
  asserts created_at = [5, 5] — UPDATE branch copies from source, INSERT branch gets
  new_version (5).
• test_update_version_tracking_merge_into_distinguishes_insert_and_update_branch (new):
  new fragment interleaves UPDATE branch rows (IDs 10, 11, source created_at = 3) and
  INSERT branch rows (IDs 500, 501); asserts created_at = [3, 5, 3, 5] to verify
  per-row correctness across both branches in the same fragment.
• test_update_version_tracking_no_row_id_meta_fallback: assertion updated from
  [1, 1, 1] to [5, 5, 5] — a fragment with no row_id_meta gets fresh stable IDs
  assigned by assign_row_ids; those IDs have no source and are INSERT branch rows,
  so created_at equals new_version.
• test_update_version_tracking_source_fragment_no_created_at_defaults_to_1 (unchanged):
  confirms that UPDATE branch rows whose source fragment has no created_at_version_meta
  still fall back to UNKNOWN (1) — the remaining reachable path through
  resolve_created_at_version.

[claude]

Claude Code Review

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[jerryjch]

cc @hamersaw

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[hamersaw]

Thanks for the fix, looks great!

[wjones127]

Thanks for working this. These changes make sense 👍