Fix HTTP/2 downstream server certificate always null

[Kevin-Ngo]

Summary

• Server certificates were always null for HTTP/2 connections because CertificateHandler stores the cert on the parent TCP channel, but the channel pool hands out child stream channels with separate attribute maps
• Try reading SERVER_CERTIFICATES from the current channel first (HTTP/1.1), then fall back to the parent channel if not found (HTTP/2 child streams)

Context on getSslTimingCallback

SslHandshakeTimingHandler serves two separate concerns:

1. The handler itself — added to the parent channel's pipeline. It records handshake duration in handlerAdded() and runs once per TCP connection.
2. getSslTimingCallback (static method) — a utility that wraps a TransportCallback and is called per-stream/per-request (e.g. in Http2NettyStreamClient). Its job is to transfer already-computed channel-level attributes (handshake duration, server certificate) into each request's RequestContext.

In HTTP/2, the channel passed to this callback is a child stream channel, but the certificate is set on the parent TCP channel by CertificateHandler. This is why the fix adds a parent-channel fallback lookup — the child stream channel will never have the SERVER_CERTIFICATES attribute set directly on it.

Testing Done

• Unit tests added (TestSslHandshakeTimingHandler)
• HTTP/1.1: cert read directly from channel
• HTTP/2: cert read from parent when child channel has none

[shivamgupta1]

LGTM