Intent of compact` carries the last-seen model id on the post-compaction usage|occurrence=1

All 1 exact test in this concept. Every test is marked by where it enters the hierarchy.

mocha:v1|namespace=vscode@05c208e9e28d8c1c723fa08f85e2b7a96092e8e5|file=vs/ 1 test
- platform/ 1 test
  - agentHost/ 1 test
    
    test/ 1 test
    
    node/ 1 test
    
    copilotAgentSession.test|title=CopilotAgentSession `/ 1 test
    
    mocha:v1|namespace=vscode@05c208e9e28d8c1c723fa08f85e2b7a96092e8e5|file=vs/platform/agentHost/test/node/copilotAgentSession.test|title=CopilotAgentSession `/compact` carries the last-seen model id on the post-compaction usage|occurrence=1 Introduced here