Add new prepare statement state MYSQL_STMT_CONNECTION_CLOSED

Since the MARIADB_STMT structure is considered opaque, there is currently no way to check whether the connection of a statement has been closed (e.g., after a reconnect or a user change) without directly accessing stmt->mysql and verifying its validity.

To address this, a new enumeration value, MYSQL_STMT_CONNECTION_CLOSED, should be added to represent the state of a prepared statement that has been closed.