[Minor] Former candidate can vote twice in one term

Nobody has claimed this yet.

Former candidates can vote twice in one term in CCF Raft. This is not a safe violation as it cannot lead to two leaders in a term but it is something we might want to address at some point (or we will need to update the spec accordingly)

Steps:
1) Node A becomes a candidate in some term, say 3
2) However, another node (node B) is elected leader of term 3
3) Node B sends an AE message to node A in term 3. Node A will step down to a follower in term 3

In the process of stepping down, `raft.h` calls `become_aware_of_new_term` and the `voted_for` field is reset, allowing the node to vote again.
https://github.com/microsoft/CCF/blob/61d6c1824f80d4714d9006b5292f2cff57473d5a/src/consensus/aft/raft.h#L1017-L1012

This report is just based of code inspection, it would be useful to generate a trace of this in practice

Start in src/consensus/aft/raft.h at the linked become_aware_of_new_term call and trace the candidate-to-follower transition. Reproduce the reported sequence with a trace if possible, confirming whether voted_for is reset and permits a second vote in the same term. Done means the behavior is confirmed and the appropriate code or specification change is identified.

Written by the indexing model from the issue text.