fix: only rewind matching session suffixes

[Aphroq]

Summary

Fixes rewind_session_items() in src/agents/run_internal/session_persistence.py so retry cleanup only removes the exact session tail that belongs to the retry being rolled back.

Previously, the helper popped session items before confirming the tail matched the rewind target, which could delete unrelated tail records. The server-tracker cleanup path could also keep popping until it found a known server item, removing the unrelated suffix and the first known server item itself. This change switches both paths to exact-suffix matching before deletion, adds best-effort restoration on mid-rewind mismatches, and restricts tracker cleanup to retry-owned tail items only.

Also adds regression coverage in tests/test_agent_runner.py for mismatched tails, server-tracker cleanup preserving unrelated items, and stripping only retry-owned local suffixes before a known server item.

Test plan

• Ran targeted rewind tests with uv run pytest tests/test_agent_runner.py -k rewind
• Ran the repository verification stack via bash .agents/skills/code-change-verification/scripts/run.sh
• Confirmed format, lint, typecheck, and full tests passed

Issue number

Closes #3089

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass