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>

dataplane-template/gatewayconfiguration.yaml

@@ -35,6 +35,18 @@ spec:
                   value: "256k"
                 - name: KONG_NGINX_HTTP_LARGE_CLIENT_HEADER_BUFFERS
                   value: "4 32k"
+                # How much of a request body nginx keeps in memory. 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 that reads the body silently stops
+                # working. That is what kept ai-proxy's log_payloads and log_statistics
+                # empty on the AI gateway, whose prompts run to 30 KB against this 8k
+                # default.
+                #
+                # Per domain, not global: 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.
+                - name: KONG_NGINX_HTTP_CLIENT_BODY_BUFFER_SIZE
+                  value: "${CLIENT_BODY_BUFFER_SIZE}"
   konnect:
     authRef:
       name: ${NAME}