Fix `Vote::Choice` variant unhandled in `cast_vote` match — non-exhaustive match or panic

[PrincessnJoy]

Description

The Vote enum in contracts/governance/src/types.rs has four variants:

pub enum Vote {
    Yes,
    No,
    Abstain,
    Choice(u32),  // ← multi-choice extension
}

However, the match vote { ... } block inside cast_vote in lib.rs only handles three of them:

match vote {
    Vote::Yes => { ... }
    Vote::No => { ... }
    Vote::Abstain => { ... }
    // Vote::Choice(n) is missing
}

Depending on whether Soroban's Rust target treats this as:

• A compile error (non-exhaustive pattern match — most likely with #[derive(...)] and Clippy match-wildcard-for-single-variants), or
• A runtime panic (if a wildcard or default arm is implicitly present)

Either outcome is unacceptable. A caller who submits Vote::Choice(0) will either be rejected at compile time or trigger undefined behavior on-chain.

Root Cause

The Vote::Choice variant was added as part of the multi-choice voting extension (see also #390) but the cast_vote logic was never updated to handle it.

Acceptance Criteria

• Add a Vote::Choice(n) arm to the cast_vote match expression
• The arm either:
  • Dispatches to the multi-choice vote accumulator (if the multi-choice feature is in scope — aligns with feat: add secret management docs and CI secret scan #390 Option A), or
  • Returns Err(ContractError::InvalidVote) with a clear error message (if multi-choice is deferred — aligns with feat: add secret management docs and CI secret scan #390 Option B)
• The match is exhaustive — no _ wildcard that silently swallows unknown variants
• Add a test for Vote::Choice on a standard Yes/No/Abstain proposal verifying it returns InvalidVote (if Option B)
• cargo build -p cosmosvote-governance succeeds (pending build(deps): bump actions/checkout from 4 to 7 #389 and feat: add secret management docs and CI secret scan #390 also being resolved)

Dependencies

• Blocked by feat: add secret management docs and CI secret scan #390 (Proposal struct missing fields) and build(deps): bump actions/checkout from 4 to 7 #389 (duplicate fn upgrade) for a clean build

Priority

High

Estimated Effort

Small