fix: skip h2c preface detection on TLS streams

[cph816]

h2c (HTTP/2 cleartext) preface detection should only run on cleartext TCP connections. On TLS, ALPN negotiates the protocol during the handshake.

When h2c is enabled and both TCP and TLS listeners share a service, preface detection runs on TLS streams too. On TLS, try_peek returns peeked=false, leaving h2c=true unconditionally. This forces all TLS connections into the HTTP/2 branch, breaking HTTP/1.1 clients.

Fix: check get_ssl_digest().is_some() to detect TLS and skip h2c detection, letting the existing ALPN check decide the protocol.

Verification:

test_h1_tls_with_h2c_enabled fails without this fix

Reproduction

1. Create a Pingora HTTP proxy service with h2c = true
2. Add both TCP and TLS listeners to the same service
3. Send an HTTP/1.1 request to the TLS port (e.g., curl --http1.1 https://...)

HTTP/2 requests to the TLS port (via ALPN) work fine.
HTTP/1.1 requests to the TCP port also work fine (h2c peek correctly detects HTTP/1.1