spanconfig: fix assertion that ensures split at tenant boundary

We added an assertion in #96014 to ensure that full reconciliation
always preserves a split point at the start of the tenant's keyspace.
The comment that introduces the assertion explains such a span config
record should be present, however, makes the assertion that such a
record is the first span config record for the tenant.

There's no real reason to make the "first record" assertion here.
In fact, that assumption isn't correct when system span configurations
are in play, as they sort before regular span configurations. I
noticed this broke `TestBackupRestoreTenantSettings`, causing a panic in
the reconciler, while workong on an unrelated patch.

This patch relaxes the "first record" assertion. We're happy as long as
it's present somewhere.

Release note: None

pkg/spanconfig/spanconfigreconciler/reconciler.go

@@ -300,6 +300,7 @@ func (f *fullReconciler) reconcile(
 	}
 
 	if !f.codec.ForSystemTenant() {
+		found := false
 		tenantPrefixKey := f.codec.TenantPrefix()
 		// We want to ensure tenant ranges do not straddle tenant boundaries. As
 		// such, a full reconciliation should always include a SpanConfig where the
@@ -309,17 +310,24 @@ func (f *fullReconciler) reconcile(
 		// tenant. Also, sql.CreateTenantRecord relies on such a SpanConfigs
 		// existence to ensure the same thing for newly created tenants.
 		if err := storeWithLatestSpanConfigs.Iterate(func(record spanconfig.Record) error {
+			if record.GetTarget().IsSystemTarget() {
+				return nil // skip over system span configurations,
+			}
 			spanConfigStartKey := record.GetTarget().GetSpan().Key
-			if tenantPrefixKey.Compare(spanConfigStartKey) != 0 {
-				return errors.AssertionFailedf(
-					"tenant prefix key %q does not match first span config start key %q",
-					tenantPrefixKey, spanConfigStartKey,
-				)
+			if tenantPrefixKey.Compare(spanConfigStartKey) == 0 {
+				found = true
+				return iterutil.StopIteration()
 			}
-			return iterutil.StopIteration()
+			return nil
 		}); err != nil {
 			return nil, hlc.Timestamp{}, err
 		}
+
+		if !found {
+			return nil, hlc.Timestamp{}, errors.AssertionFailedf(
+				"did not find split point at the start of the tenant's keyspace during full reconciliation",
+			)
+		}
 	}
 
 	return storeWithLatestSpanConfigs, readTimestamp, nil