feat(revenue_pool): expire and protect pending emergency drain proposals (#1160)

[lienhoatienphung-svg]

Closes #1160

Summary

Protects pending emergency drain proposals from remaining executable indefinitely or being silently overwritten:

1. Adds EMERGENCY_DRAIN_GRACE_PERIOD_SECONDS (24 hours) and expires_at: u64 to PendingEmergencyDrain.
2. Emits a drain_replaced event carrying (previous_drain, new_drain) whenever propose_emergency_drain replaces an existing pending proposal without prior cancellation.
3. Rejects execution after expires_at with RevenuePoolError::EmergencyDrainExpired (code 26), treats expired proposals as inactive in get_pending_emergency_drain, and provides expire_emergency_drain to purge expired proposals from instance storage with an emergency_drain_expired audit event.

Acceptance Criteria Mapping

• Second propose without cancel fails or emits replacement event: propose_emergency_drain emits drain_replaced with (previous, new) when overwriting an existing proposal (reproposal_emits_drain_replaced_event).
• Execution after expiry fails and clears the proposal: execute_emergency_drain fails with RevenuePoolError::EmergencyDrainExpired once now > drain.expires_at, get_pending_emergency_drain returns None, and expire_emergency_drain purges the expired snapshot (execute_after_expiry_fails_and_clears_proposal).
• PendingEmergencyDrain exposes expiry: pub expires_at: u64 is stored and returned by get_pending_emergency_drain.
• Tests cover both behaviours: Added unit tests in contracts/revenue_pool/src/test_emergency.rs and updated test_error_codes.rs / docs/ERROR_CODES.md.

Validation

• cargo test -p callora-revenue-pool emergency

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Payout Wallet (Base Mainnet USDC / EVM): 0x3806cd592a508d1992Ed007C2F53de251C2E7452