docs(cpex): close remaining PR #493 review nits

Final sweep to ensure every Hai Huang comment is addressed:

- cpex-policy.yaml: add a DEMO ONLY banner — the file embeds hardcoded
  client secrets (client_secret_source.kind: literal) over plaintext
  http to a local Keycloak. Addresses the "kind: literal secrets want a
  demo-only banner" nit (previously only realm-export.json was banner'd).
- cmf_a2a_test.go: TestA2AResponseBodyMod_RewritesArtifact now asserts
  the ORIGINAL secret is absent after redaction, not just that the
  redacted string is present (the reviewer's explicit ask — a partial
  rewrite would satisfy presence-only).
- README.md: correct the OPA/PDP note — the demo DOES exercise the
  embedded `cedar` PDP (scenario 05 → cpex.cedar_default_deny); only the
  external `opa(...)` / `authzen(...)` steps are doc-level BYOP examples.

Signed-off-by: Frederico Araujo <frederico.araujo@ibm.com>

Author: araujof

authbridge/authlib/plugins/cpex/README.md

@@ -152,8 +152,9 @@ checks pass; their decisions feed the same `on_deny` / `on_allow` effect lists.
 > effects, session tainting, and ordered composition with other PDPs.
 > Use the native `opa` plugin for standalone binary OPA gates; use
 > CPEX's `opa(...)` when OPA participates in a multi-PDP orchestration
-> flow. The `authzen(...)` and `cedar` steps are doc-level BYOP examples
-> not exercised in this demo.
+> flow. Note: the CPEX `opa(...)` and `authzen(...)` steps above are
+> doc-level BYOP examples — the hr-cpex demo exercises only the embedded
+> `cedar` PDP, not external OPA/AuthZEN.
 
 ## CMF / extension mapping
 

authbridge/authlib/plugins/cpex/cmf_a2a_test.go

@@ -120,6 +120,11 @@ func TestA2AResponseBodyMod_RewritesArtifact(t *testing.T) {
 	if want := "leaked [REDACTED]"; !jsonContains(t, pctx.ResponseBody, want) {
 		t.Fatalf("artifact not redacted in %s", pctx.ResponseBody)
 	}
+	// Assert the original secret is GONE, not merely that the redacted
+	// string is present (a partial rewrite would satisfy the latter).
+	if contains(string(pctx.ResponseBody), "123-45-6789") {
+		t.Fatalf("original secret still present after redaction: %s", pctx.ResponseBody)
+	}
 }
 
 func TestA2AResponseBodyMod_MultiPartFailsClosed(t *testing.T) {

authbridge/demos/hr-cpex/k8s/cpex-policy.yaml

@@ -1,5 +1,10 @@
 # CPEX runtime config for the HR demo.
 #
+# DEMO ONLY — this file embeds hardcoded client secrets
+# (client_secret_source.kind: literal) and points at a local Keycloak
+# over plaintext http. Do NOT use in production: source secrets from a
+# mounted file / Secret (kind: file) and use TLS to the IdP.
+#
 # ── How to read this file ───────────────────────────────────────────
 # The `routes:` block at the BOTTOM is the story: three tool routes,
 # one per demo act, each spelling out the authorization chain for that