fix: remove unused order type import

[AIVensk]

Closes #35. The same unused import is also reported by #36 and #37.

Remove the unused OrderTypeEnum import from websocket-event.ts. No runtime behavior changes.

Verification

• Full ESLint: zero errors and warnings across 82 files. On the unchanged base, the only error is this unused import.
• npx --no-install tsc --noEmit --incremental false: passed.
• npm run build: passed.
• Nine existing client/Nest tests: passed.
• Two selected account-order event tests using a mock HTTP executor and local WebSocket server: passed. That run uses --forceExit for pre-existing interval handles; it is not a claim that the whole socket suite passes.
• git diff --check: passed.

The broader suite's previously reproduced reconnect/order timing failures are unchanged upstream issues. Credential-dependent playground tests were excluded; no exchange transactions were performed. The import deletion was independently reviewed, and full lint, typecheck and the nine client/Nest tests were rerun successfully.

AI assistance was used for this change and verification. This is one cleanup for overlapping reports, not three distinct implementations.