[avalanche] Fix the finalized proof count being wrong when pending nodes reconnect

Summary: If a finalized proof is discarded due to missing nodes, and a node later reconnects, the proof is not polled again as expected but the finalization flag is not set properly on the peer. This diff fixes the count and add a test case that reproduces the issue. Note that only the RPC output is affected by this change.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14309