fixes issue 275 - add regression test for buy revert when creator not…

[razeprasine]

Summary

• creator-keys/tests/buy_key.rs: Added regression test test_buy_key_unregistered_creator_no_state_mutation — calls buy_key with an unregistered creator, asserts NotRegistered error, and uses capture_snapshot + assert_unchanged to confirm no state mutation
• creator-keys/tests/key_supply.rs: Added test_get_total_key_supply_increments_after_three_sequential_buys (Add unit tests for creator supply read after multiple sequential buys #274) and fixed register_creator calls to pass optional params (&None, &None, &None)

Testing

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

Checklist

• Linked issue or backlog item
• 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
• 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
• Updated docs for any changed public contract interface, read-only method, event schema, storage behavior, fee logic, or deployment workflow
• Scope stays limited to one contract concern and does not include unrelated formatting, lockfile, generated artifact, or dependency changes
  This PR has only 2 files changed with clean focused changes — it should pass CI.

Closes #274
Closes #275