Test/spam deterrence and tests

[Tukura11]

Summary

This PR adds spam-deterrence mechanisms for intent cancellation and comprehensive test coverage for the intent settlement contract,
including event assertions and end-to-end integration tests.

Changes

#63 — Add spam-deterrence for repeated intent cancellation

• Added a 10-second cooldown between consecutive cancellations per user
• Implemented `CancelCooldown` storage key to track last cancellation time
• Added `CancelCooldownNotExpired` error type
• Tests verify cooldown enforcement and independent per-user tracking

#64 — Add fuzz/property tests for compute_intent_id collision resistance

• Added 6 comprehensive tests asserting no collisions across different intent parameters
• Tests cover: different users, different amounts, different chains, different timestamps
• Batch test generates 10 intents with varying parameters to verify uniqueness

#65 — Add tests asserting exact event topics and payloads for every emitted event

• Added 15 event assertion tests covering all emitted events:
  • set_fee_recipient_updated
  • admin_transferred
  • dst_token_allowed / dst_token_disallowed
  • paused
  • solver_registered / solver_deregistered
  • bond_withdrawn
  • intent_submitted / intent_accepted / intent_filled / intent_cancelled
  • solver_slashed
  • intent_expired
• Tests verify exact topic structure and payload shape

#66 — Add end-to-end ledger-time integration test for full slash/reopen cycle

• Complete 6-step integration test exercising: submit → accept → advance time → slash → reopen → second solver fills
• Verifies intermediate IntentState transitions at each step
• Tests time-dependent branches together (slash after FILL_WINDOW, deadline reset on reopen, etc.)
• Validates solver bond penalty, active_intents counter decrements, and fund flows

Notes

• Code-only delivery: no install/build/test/scripts run during implementation
• Committer: Tukura11 (Admailo)
• All tests utilize Soroban SDK testutils for time manipulation and event inspection
• No external dependencies added

Closes #63, Closes #64, Closes #65, Closes #66

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