test(mtls): add e2e tests for mTLS client cert identity forwarding

Tests verify: mTLS proxy works with valid client cert, mTLS rejects
connections without client cert, HTTP auth plugin receives client_cn,
client_san, client_cert_fingerprint via PeerCert context propagation.

tests/e2e/scripts/auth_plugin.py

@@ -0,0 +1,27 @@
+import http.server
+import json
+import sys
+
+
+class AuthHandler(http.server.BaseHTTPRequestHandler):
+    def do_POST(self):
+        length = int(self.headers.get('Content-Length', 0))
+        body = self.rfile.read(length)
+        data = json.loads(body)
+        print(f"AUTH_REQUEST: {json.dumps(data)}", flush=True)
+        self.send_response(200)
+        self.send_header("Content-Type", "application/json")
+        self.end_headers()
+        self.wfile.write(
+            json.dumps(
+                {"ok": True, "id": data.get("username", "anonymous")}
+            ).encode()
+        )
+
+    def log_message(self, format, *args):
+        pass  # silence default logging, we use our own AUTH_REQUEST line
+
+
+if __name__ == "__main__":
+    port = int(sys.argv[1]) if len(sys.argv) > 1 else 9000
+    http.server.HTTPServer(("0.0.0.0", port), AuthHandler).serve_forever()