[avalanche] Only re-add contenders to the cache if the remote proof is present

Summary:
While debugging D18003 I discovered that the last old_quorum proof was being re-added to the cache unexpectedly. The tests would pass due to incomplete test coverage. This diff fixes both of these.

Depends on D18011

Test Plan:

ninja check
./test/functional/test_runner.py abc_p2p_avalanche_contender_voting

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18013