test: add compaction loop protection tests

32 tests covering the compactionAttempts counter state machine:
- Basic counter increment/reset behavior
- Sentry fix validation: counter resets between successful turns
- Overflow detection and compact result paths share counter
- MAX_COMPACTION_ATTEMPTS (3) loop protection
- Realistic multi-turn session scenarios
- isOverflow boundary conditions and config edge cases

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: anandgupta42