test: add assert_no_events helper and apply to failed transactions

[Johnsmichael150]

Summary

• Added a new test helper assert_no_events(&Env) in creator-keys/src/test.rs.
• Applied the helper to existing failed transaction tests to explicitly verify that no side-effect events are emitted on contract errors.
• Tests updated:
  • test_duplicate_registration_fails
  • test_buy_key_fails_if_not_registered
  • test_buy_key_insufficient_payment
  • test_set_key_price_invalid_amount

Testing

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace (All 89 tests passed)

Checklist

• Linked issue or backlog item
• Contract behavior and invariants are described clearly
• Docs updated if contract interfaces or workflows changed
• Scope stays limited to one contract concern (Event validation)

Linked Issues

Close #356
Close #321
Close #320
Close #318

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