channeldb+lnd: set invoice bucket tombstone after migration

This commit introduces the functionality to set a tombstone key
in the invoice bucket after the migration to the native SQL
database. The tombstone prevents the user from switching back
to the KV invoice database, ensuring data consistency and
avoiding potential issues like lingering invoices or partial
state in KV tables. The tombstone is checked on startup to
block any manual overrides that attempt to revert the migration.

channeldb/invoice_test.go

@@ -103,3 +103,28 @@ func TestEncodeDecodeAmpInvoiceState(t *testing.T) {
 	// The two states should match.
 	require.Equal(t, ampState, ampState2)
 }
+
+// TestInvoiceBucketTombstone tests the behavior of setting and checking the
+// invoice bucket tombstone. It verifies that the tombstone can be set correctly
+// and detected when present in the database.
+func TestInvoiceBucketTombstone(t *testing.T) {
+	t.Parallel()
+
+	// Initialize a test database.
+	db, err := MakeTestDB(t)
+	require.NoError(t, err, "unable to initialize db")
+
+	// Ensure the tombstone doesn't exist initially.
+	tombstoneExists, err := db.GetInvoiceBucketTombstone()
+	require.NoError(t, err)
+	require.False(t, tombstoneExists)
+
+	// Set the tombstone.
+	err = db.SetInvoiceBucketTombstone()
+	require.NoError(t, err)
+
+	// Verify that the tombstone exists after setting it.
+	tombstoneExists, err = db.GetInvoiceBucketTombstone()
+	require.NoError(t, err)
+	require.True(t, tombstoneExists)
+}