fix: enforce voting snapshots to prevent flash-loan attacks

[emarkees]

Closes #730

Title

Fix: Enforce strict voting snapshots to prevent flash-loan voting attacks

Description

Summary
This PR addresses a critical vulnerability detailed in #709 where users could artificially magnify their voting power by temporarily borrowing governance tokens (flash loans) within the exact transaction they cast a vote. It implements and strictly enforces the VotingSnapshot mechanism to determine voter eligibility based on balances held at the time of proposal creation, rather than instantaneous balances.

What was done

• DAO Snapshot Integration: Refactored cast_vote in governance-dao to remove the live balance query. It now cross-calls get_voting_snapshot on the token contract using the proposal's start_ledger. Votes submitted without a valid snapshot for that ledger are explicitly rejected.
• Token Snapshot Security: Hardened the take_snapshot function in governance-token to require ledger_sequence == env.ledger().sequence(). This strictly prohibits retroactive snapshot generation, effectively destroying the attack vector where an attacker flash-loans tokens and immediately attempts to backdate a snapshot.
• Testing Enhancements:
  • Updated MockGovToken in the test suite to support get_voting_snapshot for backward compatibility in standard tests.
  • Added a new integration test (test_flash_loan_vote_rejected) that accurately simulates a flash-loan scenario and asserts that the governance contract correctly panics and rejects the malicious vote.
  • Added take_snapshot strict-ledger validations in governance-token tests.

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