gateway: use 191-char prefix index on aa_instance_heartbeats.instance_id

The unique index on `instance_id` (VARCHAR(255), utf8mb4) overflowed
MySQL InnoDB's 767-byte key length limit (255 * 4 = 1020 bytes),
causing `CREATE UNIQUE INDEX ... ON aa_instance_heartbeats(instance_id)`
to fail with "Specified key was too long" during gateway startup and
aborting the migration.

Switch the MySQL DDL to a 191-char prefix index, which keeps the index
key under 767 bytes (191 * 4 = 764) while remaining longer than any
realistic instance_id value. SQLite path is unaffected.

Signed-off-by: Jiale Zhang <xinjian.zjl@alibaba-inc.com>

Author: Jiale Zhang

trustee-gateway/internal/persistence/storage/database.go

@@ -238,7 +238,9 @@ func (d *Database) migrateAAInstanceHeartbeatIndexMySQL() error {
 		logrus.Info("Cleaned up duplicate instance_id records in aa_instance_heartbeats")
 	}
 
-	if err := d.DB.Exec("CREATE UNIQUE INDEX idx_aa_heartbeat_instance_id ON aa_instance_heartbeats(instance_id)").Error; err != nil {
+	// Use a 191-char prefix to stay under MySQL InnoDB's 767-byte index key
+	// limit when instance_id is VARCHAR(255) utf8mb4 (4 bytes/char → 1020 bytes).
+	if err := d.DB.Exec("CREATE UNIQUE INDEX idx_aa_heartbeat_instance_id ON aa_instance_heartbeats(instance_id(191))").Error; err != nil {
 		return fmt.Errorf("failed to create unique index on instance_id: %w", err)
 	}