fix(compaction): persist serialized tail separately

[rekram1-node]

Closes: #14332

Summary

• Restores the compaction summarization request to only include the selected head history.
• Serializes the preserved recent tail into a separate synthetic user message after the generated summary.
• Marks that synthetic tail so the prompt loop does not treat it as a new user request.

Context

This fixes a regression introduced by #26830. That change correctly moved away from replaying raw retained tail messages, but it also folded the serialized tail into the summarization request itself. As a result, recent tail content influenced summary generation instead of being persisted as post-compaction conversation context.

The corrected flow keeps the summarization request unchanged, saves the model-generated summary as-is, and stores the serialized tail separately for future context.

Testing

• bun test test/session/compaction.test.ts

[rekram1-node]

/review

[rekram1-node]

/review

[rekram1-node]

/review

[rekram1-node]

/review

[github-actions]

lgtm