test(nestjs-gateway): restore integration coverage

[TorinAsakura]

Task

• Closes [Feature] Restore gateway integration coverage #410

How to verify

Main scenario

1. Context: gateway package integration contour
   Action: run integration tests for the package
   Expected result: GraphQL HTTP requests and websocket subscriptions pass in a non-empty suite

Additional scenario

1. Context: gateway package local validation
   Action: run package lint, typecheck, unit tests, build, and diff check
   Expected result: all checks pass

Proofs

• Local verification

mise x node@22.22.3 -- yarn format packages/nestjs-gateway
mise x node@22.22.3 -- yarn lint packages/nestjs-gateway
mise x node@22.22.3 -- yarn typecheck packages/nestjs-gateway
mise x node@22.22.3 -- yarn test integration packages/nestjs-gateway
mise x node@22.22.3 -- yarn test unit packages/nestjs-gateway
mise x node@22.22.3 -- yarn workspace @atls/nestjs-gateway build
git diff --check

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