[avalanche] Optionally assert a new proof orphan state

Summary:
This is a small simplification extracted from D9691.

Depends on D9691.

Test Plan:

ninja check-functional
./test/functional/test_runner.py p2p_inv_download

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9692