test: add quorum boundary edge case tests for governance proposals

[owohdora-stack]

Add tests for quorum edge cases

Closes #1957

Summary

Adds 6 deterministic unit tests to src/test_governance.rs that lock down the quorum boundary logic in Governable::finalize_proposal (governance.rs:262):

let total = proposal.votes_for.saturating_add(proposal.votes_against);
let passed = total >= Self::quorum() && proposal.votes_for > proposal.votes_against;

The test harness uses a quorum of 3.

Tests added

Test	Total votes	For	Against	Expected
finalize_proposal_passes_when_exactly_at_quorum_with_majority_for	3 (at quorum)	2	1	Passed
finalize_proposal_rejects_when_exactly_at_quorum_with_majority_against	3 (at quorum)	1	2	Rejected
finalize_proposal_rejects_when_one_vote_below_quorum_even_with_majority_for	2 (below quorum)	2	0	Rejected
finalize_proposal_rejects_when_one_vote_below_quorum_with_split_votes	2 (below quorum)	1	1	Rejected
finalize_proposal_passes_when_one_vote_above_quorum_with_majority_for	4 (above quorum)	3	1	Passed
finalize_proposal_rejects_when_one_vote_above_quorum_but_tied	4 (above quorum)	2	2	Rejected

Coverage

• Happy path: Quorum met with majority for passes (at and above threshold).
• Sad paths: Quorum not met (even with majority for), quorum met but tied, quorum met with majority against — all correctly reject.

Verification

• All tests follow existing patterns: use setup(), proposal_id(), cast_vote(), finalize_proposal() helpers.
• Deterministic: no Date.now() / Math.random() — uses injected Address::generate and fixed ledger sequences.
• #![no_std] discipline maintained: only soroban_sdk primitives used.
• Test names are assertive and descriptive.
• Only test_governance.rs modified; no unrelated refactors.

Note

The crate has pre-existing compilation errors on main (duplicate definitions, missing enum variants, unclosed delimiter in settlement.rs) that prevent cargo test from running. These are unrelated to quorum logic and tracked separately.

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