fix(governance): validate snapshot_ledger is strictly in the past (closes #504)

[Ranjeet2063]

Summary

Enforces invariant validation in Soroban smart contract governance proposal creation ensuring that snapshot_ledger is strictly earlier than the current ledger sequence (snapshot_ledger < env.ledger().sequence()).

Key Fixes & Tests

1. Contract Invariant:

   • In create_proposal(), panics with "snapshot_ledger must be earlier than the current ledger"" when snapshot_ledger >= env.ledger().sequence()`.
   • Prevents retroactive flash-loan/just-in-time token minting voting attacks by guaranteeing voting power strictly references a completed past ledger snapshot.

2. Automated Unit Verification:

   • Added and fixed test_create_proposal_rejects_current_or_future_snapshot_ledger asserting rejection on current/future ledger sequences.
   • Updated pause boundary tests (test_create_proposal_fails_while_paused, test_vote_fails_while_paused, test_execute_proposal_fails_while_paused) with consistent historical ledger snapshots.
   • Verified 100% green with cargo test.

Closes #504

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