test(node): unskip transaction rollback and add loadFile parity coverage

[pbbadenhorst]

Summary

The Node test suite skipped the transaction rollback test with a comment
claiming transaction() "doesn't properly return a promise." That claim is
inaccurate — NodeDatabase.transaction() is async and already does
BEGIN / await fn() / COMMIT, with a catch that runs ROLLBACK and
rethrows. Unskipping the test as written passes; the skip comment was stale.

This closes the parity-coverage gaps in the Node test suite.

Changes

• Unskip Transaction rollback and tighten it: assert the inner error is
  rethrown and that the transient row is absent after rollback — not just an
  unchanged row count, which could mask a partial commit.
• Drop the stale // Skipped: ... comment from Transaction commit; the test
  was already running, only mislabelled.
• Add a Load SQL file test exercising db.loadFile(). No test previously
  covered that path.

Test plan

• cd node && npm test — 18/18 pass
• cd node && npx tsc --noEmit — clean

[bernard-jigx]

Small fix re-enabling tests and adding a load file test. This was picked up previously on another PR, but did not what to add it to that one as noise.

[ospfranco]

sure, thanks!