test: advance DuckDB fast-path fixture timestamps

DuckDB incremental push tests rewrite local sessions after the initial push watermark has advanced. Leaving fixture LocalModifiedAt values at their original timestamps meant those tests could skip the session selection path they were intended to exercise.

Set fresh local modification times in the rewrite helpers so the follow-up incremental pushes deterministically cover the message-id repair and lifecycle fallback paths.

Author: cpcloud

internal/duckdb/sync_fastpath_test.go

@@ -424,6 +424,8 @@ func rewriteLocalMessagesPreservingContent(
 	require.NotNil(t, sess)
 	msgs, err := local.GetAllMessages(ctx, sessionID)
 	require.NoError(t, err)
+	modifiedAt := time.Now().UTC().Format(localSyncTimestampLayout)
+	sess.LocalModifiedAt = &modifiedAt
 	sess.MessageCount = len(msgs)
 	_, err = local.WriteSessionBatchAtomic([]db.SessionBatchWrite{{
 		Session:         *sess,
@@ -497,6 +499,8 @@ func writeFastPathLifecycleSession(
 		sessionID, "alpha", "lifecycle first",
 		"2026-01-21T00:00:00.000Z", len(messages),
 	)
+	modifiedAt := time.Now().UTC().Format(localSyncTimestampLayout)
+	sess.LocalModifiedAt = &modifiedAt
 	write := db.SessionBatchWrite{
 		Session:         sess,
 		Messages:        messages,