feat: add automatic HTTP/2 and HTTP/1.1 negotiation

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Summary

• restore HTTP/1.1 request handling alongside the existing HTTP/2 transport
• auto-detect cleartext HTTP/2 prior-knowledge preface and otherwise fall back to HTTP/1.1
• negotiate TLS with ALPN, preferring h2 and falling back to http/1.1
• serialize responses using the protocol selected for the request
• add a real socket regression test for HTTP/1.1 fallback

This keeps the existing server API unchanged. HTTP/2 remains preferred when the client supports it; ordinary browser HTTP requests such as Swagger UI can use HTTP/1.1 automatically.