fix: replace ConversationBufferLocks map with bounded cache

[me0106]

Summary

• Replace the unbounded session lock map with a bounded Caffeine cache.
• Expire inactive lock sentinels after access to avoid unbounded growth.

Test Plan

• JAVA_HOME=/Library/Java/JavaVirtualMachines/temurin-25.jdk/Contents/Home /opt/homebrew/bin/mvnd -pl memind-core -am test

Closes #89