fix: bound client suite concurrency - #509

Summary

• bound client-suite execution concurrency for timing-sensitive SSE checks
• keep the existing scenario assertions and lifecycle behavior intact

Why

Issue #500 identified that the client suite could create timing interference when too many checks ran concurrently. A bounded runner makes the test behavior more stable without changing production code.

Validation

• pre-push formatting check passed
• pre-push test passed: 48 files, 629 tests
• npm run typecheck
• npm run build
• npm run lint
• git diff --check

Fixes #500