test: add assert_event helper for contract event field assertions (#559)

[EneGab]

Summary

• Closes Add helper for asserting contract event fields in test assertions #559 by introducing a shared test helper assert_event(env, contract_id, topic, expected_fields) to eliminate repetitive boilerplate across event test assertions.
• Added assert_event to creator-keys/tests/contract_test_env/mod.rs:
  • Filters events in env.events().all() by both emitting contract address (contract_id) and canonical event topic at index 0 (topic).
  • Deserializes event payloads into generic contract types implementing TryFromVal<Env, Val> + PartialEq + core::fmt::Debug.
  • Produces clear, readable panic messages showing the queried topic, expected fields, and actual emitted fields on mismatch, as well as an explicit failure message when no matching event was found.
• Added dedicated unit tests for the helper in creator-keys/tests/assert_event.rs verifying exact match, topic filtering, contract ID filtering, field value mismatch messages, and behavior when no events are emitted.
• Refactored existing event-emitting tests to adopt assert_event:
  • creator-keys/tests/contract_initialization_event.rs (test_initialization_event_emitted_on_first_set_fee_config) for ContractInitializedEvent.
  • creator-keys/tests/creator_registered_event_fields.rs (test_creator_registered_event_all_four_fields_present_in_one_registration) for CreatorRegisteredEvent.

Testing

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace

Checklist

• Linked issue or backlog item (Add helper for asserting contract event fields in test assertions #559)
• Added or updated creator-keys unit/integration tests for every changed contract behavior, including failure paths for new or reachable ContractError variants
• Ran cargo fmt --all -- --check, cargo clippy --workspace --all-targets -- -D warnings, and cargo test --workspace, or explained exactly why a command was not run
• Reviewed persistent storage changes against docs/storage-key-invariants.md; any storage layout change includes a migration/backward-compatibility note (N/A - no storage layout modifications)
• Confirmed event names, topic order, payload field order, and field meanings remain compatible with docs/contract-event-conventions.md, or documented the breaking change and versioning plan (N/A - event schemas unchanged)
• Updated docs for any changed public contract interface, read-only method, event schema, storage behavior, fee logic, or deployment workflow (N/A - test helper only, no contract interface changes)
• Scope stays limited to one contract concern and does not include unrelated formatting, lockfile, generated artifact, or dependency changes

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