Make the request body buffer a per-domain setting

ai-proxy's log_payloads and log_statistics produced nothing on the AI
gateway despite being enabled. The cause was not the plugin's own
max_request_body_size but nginx's client_body_buffer_size, left at its 8k
default: past it the body goes to a temp file, and
kong.request.get_raw_body() returns nil for a body on disk, so any plugin
reading the body silently stops working. Prompts on that gateway run to 30
KB. Measured, not guessed — request.size was 30369 on the last call.

Expose it as ${CLIENT_BODY_BUFFER_SIZE} with the 8k default every domain
keeps, rather than raising it globally. The buffer is allocated per
connection carrying a body, so a value that is free on a gateway seeing 18
requests a day is not free on one serving real traffic.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

kong-platform/domain-defaults/dataplane-template-defaults.yaml

@@ -21,6 +21,9 @@ data:
   CORS_ORIGINS: https://*.sttlab.local
   RATE_LIMIT_MINUTE: "300"
   REQUEST_MAX_BODY_SIZE: "10" # MB
+  # nginx default. Domains that need to introspect larger bodies raise it
+  # in their own Kustomization; see the AI gateway.
+  CLIENT_BODY_BUFFER_SIZE: 8k
   # Tempo (OTLP-native), ns observability, cluster-wide — shared by every
   # domain, not domain-specific. Replaced Jaeger on 2026-08-25: Tempo is the
   # trace backend of the infra-observability stack, so its span metrics feed