fix: avoid duplicate content-length on empty responses

[MukundaKatta]

Closes #147

Summary

• route empty responses with a tracked numeric content-length through endWithoutBody()
• reuse the same ending helper for the immediate and backpressure retry paths
• add a regression test for send() with a custom content-length and no body

Verification

• npm test -- --runTestsByPath src/http/core/response.spec.ts
• npm run build
• npm run lint -- --quiet
• git diff --check

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[VikramAditya33]

LGTM, Thank you