fix(ingestion): remove nested transaction in upsert_mr_file_changes

drain_mr_diffs in orchestrator.rs already wraps each MR diff store
in an unchecked_transaction (alongside job completion and watermark
update). upsert_mr_file_changes was also starting its own inner
transaction via conn.unchecked_transaction(), causing every call to
fail with "cannot start a transaction within a transaction".

Remove the inner transaction management from upsert_mr_file_changes
so it operates on whatever Connection (or Transaction deref'd to
Connection) the caller provides. The caller in drain_mr_diffs owns
the transaction boundary. Standalone callers (tests, future direct
use) auto-commit each statement, which is correct for their use case.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ingestion/mr_diffs.rs

@@ -19,20 +19,22 @@ fn derive_change_type(diff: &GitLabMrDiff) -> &'static str {
 
 /// Replace all file change records for a given MR with the provided diffs.
 /// Uses DELETE+INSERT (simpler than UPSERT for array replacement).
+///
+/// Does NOT manage its own transaction — the caller is responsible for
+/// wrapping this in a transaction when atomicity with other operations
+/// (job completion, watermark update) is needed.
 pub fn upsert_mr_file_changes(
     conn: &Connection,
     mr_local_id: i64,
     project_id: i64,
     diffs: &[GitLabMrDiff],
 ) -> Result<usize> {
-    let tx = conn.unchecked_transaction()?;
-
-    tx.execute(
+    conn.execute(
         "DELETE FROM mr_file_changes WHERE merge_request_id = ?1",
         [mr_local_id],
     )?;
 
-    let mut stmt = tx.prepare_cached(
+    let mut stmt = conn.prepare_cached(
         "INSERT INTO mr_file_changes (merge_request_id, project_id, old_path, new_path, change_type) \
          VALUES (?1, ?2, ?3, ?4, ?5)",
     )?;
@@ -56,10 +58,6 @@ pub fn upsert_mr_file_changes(
         inserted += 1;
     }
 
-    // Drop the prepared statement before committing the transaction.
-    drop(stmt);
-    tx.commit()?;
-
     if inserted > 0 {
         debug!(inserted, mr_local_id, "Stored MR file changes");
     }