refactor: keep segment runtime context transient

[starboyate]

Summary

• introduce a transient SegmentRuntimeContext for conversation extraction so runtime-only message context no longer leaks into durable rawdata metadata
• update chunking, rawdata processing, item extraction, and self-verification to read timing and user context from runtime context instead of persisted messages
• keep JDBC/MyBatis segment persistence backward compatible while explicitly stripping runtime context from stored segment payloads

Test Plan

• mvn -pl memind-core,memind-plugins/memind-plugin-jdbc,memind-plugins/memind-plugin-spring-boot-starters/memind-plugin-mybatis-plus-starter -am test