Document HTTP/2 client production hardening; add the defenses to the sample

Expand the HTTP/2 Java client guide (http2-java-client.xml) with a Production
Hardening section covering the connection-pool defenses a long-lived HTTP/2 client
needs to survive upstream restarts and dropped network paths: stale-connection
validation + TTL + idle/expired eviction, TCP keepalive tuning (with the network
idle-timeout and Java/OS caveats), bounded retry for stale pooled connections
(idempotency + no-partial-output rules), and fail-fast on the response status via
AbstractBinResponseConsumer.

Update the Http2JsonClient sample to apply all four: validateAfterInactivity + TTL,
evictExpired/evictIdle, the TCP keepalive triplet, and a retryable-connection
classifier with a zero-bytes-written retry gate. Align httpcore5-h2 to 5.4.3.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: robertlazarski