fix(compaction): handle immutable session history

[alexk-dev]

Summary

• fix /compact so compaction no longer mutates session.getMessages() in place
• switch the mutation path to AgentSession.mutableMessages() before clearing/rebuilding history
• add a regression test covering immutable session message lists

Root Cause

CompactionOrchestrationService assumed session.getMessages() was always mutable and called clear() directly. When the session history list was immutable, /compact failed with java.lang.UnsupportedOperationException.

Testing

• ./mvnw -Dmaven.gitcommitid.skip=true -Dtest=CompactionOrchestrationServiceTest,CommandRouterTest test

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
85.7% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud