Add MySQL schema for SPRING_AI_CHAT_MEMORY

[hpoettker]

Problem statement

Although there is a MysqlChatMemoryRepositoryDialect for the JDBC chat memory repository, auto-configuration of the repository currently fails to initialize the table in MySQL databases.

When schema initialization is enabled, the application fails with the message:

Error creating bean with name 'jdbcChatMemoryScriptDatabaseInitializer' defined in class path resource [org/springframework/ai/model/chat/memory/repository/jdbc/autoconfigure/JdbcChatMemoryRepositoryAutoConfiguration.class]: No schema scripts found at location 'classpath:org/springframework/ai/chat/memory/repository/jdbc/schema-mysql.sql'

Among other schema files, there is a schema-mariadb.sql, but this file doesn't work with MySQL as MySQL doesn't support CREATE INDEX with IF NOT EXISTS.

Proposed change

This pull request

• adds a schema-mysql.sql,
• renames the existing test JdbcChatMemoryRepositoryMysqlIT, which tests against MariaDB, to JdbcChatMemoryRepositoryMariaDbIT,
• adds a new JdbcChatMemoryRepositoryMysqlIT, which tests against MySQL.

Remarks on the MySQL schema

In the MySQL schema, I've used the column type ENUM instead of a check constraint. A check constraint would also work in recent MySQL releases, as they have been added in MySQL 8.0.16. But using an ENUM extends compatibility even back to MySQL 5.7 and is also a more direct representation of the Java Enum MessageType.

[ilayaperumalg]

@hpoettker Thanks for the PR adding the schema for MySQL JDBC chat memory repository! The changes look good. Rebased and merged as 9f58520. Also back-ported into 1.0.x as 148bf33