Backport session history edge-case coverage for tool-call boundaries

[bitifirefly]

Summary

• backport the broader session-history regression coverage from upstream around legal tool-call boundaries
• add edge-case tests for:
  • preserving complete tool-call/result groups after trimming
  • behavior when last_consolidated is non-zero
  • no-tool plain conversations
  • all-orphan prefix stripping
  • empty sessions
  • windows that cut mid tool group
• this validates existing Session.get_history() behavior against additional boundary scenarios without changing runtime logic

Testing

• ruff check tests/test_session_manager_history.py
• pytest -q tests/test_session_manager_history.py
• pytest -q (440 passed)