payments/migration1: add migration test suite and helpers

Add test helpers plus sql_migration_test coverage for KV→SQL migration.
Basic migration, sequence ordering, data integrity, and feature-specific cases
(MPP/AMP, custom records, blinded routes, metadata, failure messages). Also
cover duplicate payment migration to payment_duplicates, including missing
attempt info to ensure terminal failure is recorded.

This gives broad regression coverage for the migration path and its edge-cases.

payments/db/migration1/test_harness.go

@@ -0,0 +1,26 @@
+package migration1
+
+import (
+	"testing"
+
+	"github.com/lightningnetwork/lnd/lntypes"
+)
+
+// TestHarness provides implementation-specific test utilities for the payments
+// database. Different database backends (KV, SQL) have different internal
+// structures and indexing mechanisms, so this interface allows tests to verify
+// implementation-specific behavior without coupling the test logic to a
+// particular backend.
+type TestHarness interface {
+	// AssertPaymentIndex checks that a payment is correctly indexed.
+	// For KV: verifies the payment index bucket entry exists and points
+	// to the correct payment hash.
+	// For SQL: no-op (SQL doesn't use a separate index bucket).
+	AssertPaymentIndex(t *testing.T, expectedHash lntypes.Hash)
+
+	// AssertNoIndex checks that an index for a sequence number doesn't
+	// exist.
+	// For KV: verifies the index bucket entry is deleted.
+	// For SQL: no-op.
+	AssertNoIndex(t *testing.T, seqNr uint64)
+}