Verify the 413 body as JSON and use engine terminology in the docs

The oversized-body test now unmarshals the 413 response and asserts
the exact error payload instead of substring-matching, so the
response can never silently stop being valid JSON.

docs/proxy-request-hardening.mdx uses engine where the prose means
the engine address rather than the set-local-backend wire name, and
the sequence diagram shows the 413 payload as the JSON it is.

Signed-off-by: Mallikh Kaula <mallikh@users.noreply.github.com>

Author: MALLIKH

docs/proxy-request-hardening.mdx

@@ -57,7 +57,7 @@ sequenceDiagram
 The proxy never holds more than cap+1 bytes per in-flight request body, so a
 malicious or buggy client can no longer exhaust proxy memory.
 
-## 2. The local backend must be loopback
+## 2. The local engine address must be loopback
 
 ### The problem
 
@@ -71,7 +71,7 @@ approved.
 
 ### The fix
 
-The unified ingress enforces the invariant where the backend is stored:
+The unified ingress enforces the invariant where the engine address is stored:
 `setLocalBackend()` in `ingress.go` refuses a non-loopback host outright and
 returns an error, so a bad value never reaches the dial path at all.
 
@@ -85,7 +85,7 @@ flowchart TD
     A[broker: set-local-backend] --> B{host empty?}
     B -->|yes| C[store, default 127.0.0.1 at dial time]
     B -->|no| D{ParseIP → IsLoopback?}
-    D -->|yes| E[store backend]
+    D -->|yes| E[store engine address]
     D -->|no| F[refuse: return error<br/>nothing stored]
 ```
 
@@ -130,7 +130,7 @@ longer downgrade to an unauthenticated connection in the meantime.
 ## Validation
 
 - `proxy_hardening_test.go` covers all three fixes: over-cap and at-cap
-  bodies, the 413 response shape, loopback/non-loopback backend hosts, and
+  bodies, the 413 response shape, loopback/non-loopback engine hosts, and
   the fail-closed dial error. Run with `go test -race ./...` from the
   component directory.
 - No JSON-RPC method or payload changed, so no contract regeneration was

services/nvpair-proxy/proxy_hardening_test.go

@@ -4,6 +4,7 @@
 package main
 
 import (
+	"encoding/json"
 	"io"
 	"net/http"
 	"net/http/httptest"
@@ -67,8 +68,12 @@ func TestHandleHTTPRejectsOversizedBody(t *testing.T) {
 	if ct := rec.Header().Get("Content-Type"); ct != "application/json" {
 		t.Errorf("Content-Type = %q, want application/json", ct)
 	}
-	if !strings.Contains(rec.Body.String(), "request body exceeds") {
-		t.Errorf("413 body = %q, want it to name the limit", rec.Body.String())
+	var payload map[string]string
+	if err := json.Unmarshal(rec.Body.Bytes(), &payload); err != nil {
+		t.Fatalf("413 body is not valid JSON: %v (body = %q)", err, rec.Body.String())
+	}
+	if payload["error"] != "request body exceeds 32 MiB limit" {
+		t.Errorf("413 error = %q, want %q", payload["error"], "request body exceeds 32 MiB limit")
 	}
 }