Add benchmark: governance-voting vote-cast gas cost

[christabel888]

Context: There is a single shared benches crate in the workspace (benches/benches/contract_benchmarks.rs), not per-crate bench directories.

Where:

• benches/benches/contract_benchmarks.rs -- add a benchmark function for the governance-voting contract's vote-cast path

What to do:

• Add a Criterion (or existing harness, match what's already in contract_benchmarks.rs) benchmark measuring CPU/instruction cost for the call above.
• Record a baseline number in the PR description so future gas-regression tooling (see the CI issue in this batch) has something to diff against.

Suggested approach:

1. Look at whatever benchmark(s) already exist in benches/benches/contract_benchmarks.rs for the harness pattern (Criterion or otherwise) before adding a new one.
2. Benchmark the specific governance-voting call path named in this issue in isolation -- set up state once, then measure only the call itself, not the setup.
3. Run it against main first to get a baseline number, then again on your branch if you're also touching this crate's logic elsewhere, and report both in your PR.
4. Use the workspace's [profile.bench] (opt-level = 3) -- don't benchmark against a debug build, the numbers are meaningless.

Watch out for:

• Soroban's real cost metric is CPU instructions / resource fees, not wall-clock time -- if the existing harness measures wall-clock only, flag that as a limitation rather than presenting it as a gas-cost benchmark.
• Benchmarks that depend on random/varying input sizes need a fixed seed or fixed input, or your baseline comparison is noise.

Definition of done:

• Benchmark added and runs via cargo bench (uses the workspace [profile.bench], opt-level 3)
• Baseline cost recorded in the PR