fix: remove setStatus(OK) calls to comply with OTel spec

[newbee1939]

Fixes #126

Remove setStatus(OK) calls from finalizeResponseSpanHook and finalizeNotFoundSpanHook to comply with the OTel spec, which states instrumentation libraries should leave span status as Unset unless there is an error.

• Only set SpanStatusCode.ERROR for 5xx responses
• Use reply.statusCode instead of hardcoded 404 in finalizeNotFoundSpanHook

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct