Fix atomic conversation buffer drains for persistent stores

[starboyate]

Summary

• Fixes router: PostgresqlConversationBuffer.drain() has TOCTOU race — duplicate processing under concurrency #88 by making persistent pending conversation buffer drains atomic across JDBC PostgreSQL, MySQL, SQLite, and MyBatis Plus.
• Replaces select-then-mark drain paths with dialect-appropriate claim-and-return operations while preserving recent conversation history.
• Repairs MySQL DDL/schema bootstrap for legacy multimodal raw data columns/indexes and procedure-safe init scripts.

Test Plan

• mvn -pl memind-plugins/memind-plugin-jdbc/memind-plugin-jdbc-core -Dtest=SqlScriptRunnerTest,JsonCodecTest test
• mvn -pl memind-plugins/memind-plugin-jdbc/memind-plugin-jdbc-sqlite -am -DfailIfNoTests=false test
• mvn -pl memind-plugins/memind-plugin-jdbc/memind-plugin-jdbc-postgresql -am -Dtest=PostgresqlConversationBufferConcurrencyTest -DfailIfNoTests=false -DfailIfNoSpecifiedTests=false test
• mvn -pl memind-plugins/memind-plugin-jdbc/memind-plugin-jdbc-mysql -am -Dtest=MysqlConversationBufferConcurrencyTest,MysqlStoreSchemaBootstrapIntegrationTest -DfailIfNoTests=false -DfailIfNoSpecifiedTests=false test
• mvn -pl memind-plugins/memind-plugin-spring-boot-starters/memind-plugin-mybatis-plus-starter -am -Dtest=MybatisPlusConversationBufferTest#autoConfiguredPendingBufferDrainsPreexistingBatchOnceConcurrently,DialectConversationBufferAtomicOperationsTest,MemoryStoreDdlMysqlIntegrationTest -DfailIfNoTests=false -DfailIfNoSpecifiedTests=false test
• git diff --check
• git diff --cached --check