[release/6.x] Backport pre-vote capability (#7374, #7375, #7404, #7409, #7438, #7445, #7458)

[cjen1-msft]

Primarily (#7404, #7409, #7438, #7445, #7458), but also (#7374, #7375).

Per the migration plan for enabling pre-vote, we should first make 6.X capable of receiving and responding to pre-vote messages, and then we will enable pre-vote by default in 7.x.

This includes the backport of the CheckQuorum changes (#7374, #7375), as otherwise the CI for tracing will likely fail.

A slightly safer version of this backport, is to separate out the CheckQuorum changes, and then backport just the changes to the recv_request_vote without the changes for if a replica is a pre-vote candidate.
However the current PR should be a no-op for existing services.

[Copilot]

Pull Request Overview

This PR adds support for the PreVote optimization in the Raft consensus protocol. PreVote prevents isolated followers from disrupting the cluster by requiring them to first become PreVoteCandidates and receive a quorum of speculative pre-votes before becoming actual Candidates and incrementing the term.

Key changes:

• Introduces a new PreVoteCandidate leadership state and three PreVote status levels (Disabled, Capable, Enabled)
• Updates the CheckQuorum logic to require quorum in every active configuration (not just any configuration)
• Adds TLA+ trace validation support for PreVote scenarios and CLI integration for scenario-based testing

Reviewed Changes

Copilot reviewed 26 out of 26 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tla/tlc.py	Adds --scenario CLI option for generating and validating traces from scenario files
tla/consensus/ccfraft.tla	Implements PreVote state machine, adds PreVoteCandidate state and related actions
tla/consensus/Traceccfraft.tla	Updates trace validation to support PreVote messages and state transitions
src/consensus/aft/raft.h	Implements PreVote logic in C++, refactors CheckQuorum, handles PreVote messages
src/consensus/aft/raft_types.h	Adds ElectionType enum (PreVote/RegularVote) to request vote messages
src/consensus/aft/impl/state.h	Adds pre_vote_enabled field to State struct
tests/raft_scenarios/.	New test scenarios for PreVote and CheckQuorum behavior
doc/architecture/consensus/index.rst	Documents PreVote extension and migration strategy
CHANGELOG.md	Documents PreVote support and CheckQuorum fix

Comments suppressed due to low confidence (1)

tla/consensus/Traceccfraft.tla:65

• Missing validation of the isPreVote field in IsRequestVoteResponse. This predicate should validate msg.isPreVote against the logline packet's election_type field, similar to how IsRequestVoteRequest handles it. This omission could lead to incorrect trace validation when PreVote responses are processed.

IsRequestVoteResponse(msg, dst, src, logline) ==
    /\ IsHeader(msg, dst, src, logline, RequestVoteResponse)
    /\ msg.voteGranted = logline.msg.packet.vote_granted

[eddyashton]

Converting this to draft - it shouldn't be merged to 6.x without #7445 being included as well.