fix(knowledge): clear stale error_message on reprocessing and finalize

[oscarlius]

Description

Once a knowledge item fails, error_message stays set on the row. When the item is later reprocessed or driven through the finalizing pipeline, nothing clears that column, so the UI keeps displaying an outdated failure message on items that are processing again or have already completed successfully.

This PR clears error_message at exactly the transitions where a stale failure becomes misleading:

• SetFinalizing — clears it in the same atomic UPDATE that flips parse_status to finalizing
• FinalizeSubtask — clears it in the same atomic UPDATE that promotes finalizing → completed (so a successfully promoted row can’t surface an error from a previous attempt)
• ProcessDocument — clears knowledge.ErrorMessage when the row flips back to processing for a new attempt

Both repository transitions clear the column inside the same conditional UPDATE that guards on the current status, so the atomicity/race properties of the existing code are unchanged.

Type of Change

• 🐛 Bug fix

Related Issue

N/A

Testing

• New SQLite-backed regression test TestSetFinalizingAndFinalizeSubtask_ClearStaleErrorMessage: seeds a stale error_message, asserts SetFinalizing clears it, re-seeds, and asserts the FinalizeSubtask promotion clears it again.
• go test ./internal/application/repository/ -count=1 — passes
• gofmt -l on changed files — clean; git diff --check — passes
• golangci-lint run --new-from-rev=upstream/main ./internal/... — 0 issues

Checklist

• git diff --check origin/main...HEAD passes
• Changed source files are formatted
• Targeted tests for the changed packages/components pass
• Diff-scoped lint passes where applicable
• Full-repository checks were run, or any unrelated/environment-dependent failures are documented above
• Self-reviewed the code
• Added/updated tests covering the change
• Updated related documentation (method comment documents the clearing semantics)
• Breaking changes are clearly called out in the description above