fix(sessions): raise InvocationNotFoundError instead of bare ValueError on rewind

[ecanlar]

Summary

rewind_session (in _rewind_utils.py) raises a bare ValueError with a hardcoded message when rewind_before_invocation_id matches no event in the session:

raise ValueError(f"Invocation ID not found: {rewind_before_invocation_id}")

Callers of Runner.rewind_async have no stable way to distinguish this specific failure from any other ValueError, short of matching against that exact message string. That string is not part of any documented contract, so a future wording change to it would silently break any caller relying on it — with no test able to catch the break, since a hand-constructed ValueError in a test can't detect a wording drift in the library's own raised message.

This mirrors the situation SessionNotFoundError was already introduced to solve for the "session not found" case.

Change

• Add InvocationNotFoundError (errors/invocation_not_found_error.py), a ValueError subclass — for backward compatibility — following the exact pattern of SessionNotFoundError.
• rewind_session now raises InvocationNotFoundError instead of the bare ValueError, keeping the same message text.
• Document the new exception in Runner.rewind_async's docstring (Raises: section).
• Update the existing unit test asserting on this path to check the new type.

Test plan

• pytest tests/unittests/sessions/test_rewind_utils.py tests/unittests/runners/test_runner_rewind.py — all 9 tests pass.
• pyink, isort, ruff check on the changed files — no issues.
• Backward compatible: InvocationNotFoundError is still a ValueError, so any existing except ValueError handling keeps working unchanged.