plasma: add testnet compose + assets (reth + plasma-consensus)

Canonicalize plasma-testnet so the tracked compose adopts the same
volume/service names as the de-32 hand-written override (rpc_plasma-
testnet-reth-pruned-trace-{execution,consensus,jwt}). Also refresh
mainnet observer sed to be chain-agnostic.

compose_registry.json

@@ -8520,6 +8520,24 @@
       "plasma-mainnet-reth-pruned-trace-jwt"
     ]
   },
+  {
+    "chain": "testnet",
+    "client": "reth",
+    "compose_file": "plasma/reth/plasma-testnet-reth-pruned-trace",
+    "features": [
+      "trace"
+    ],
+    "network": "plasma",
+    "node": "plasma-consensus",
+    "relay": null,
+    "stack": null,
+    "type": "pruned",
+    "volumes": [
+      "plasma-testnet-reth-pruned-trace-consensus",
+      "plasma-testnet-reth-pruned-trace-execution",
+      "plasma-testnet-reth-pruned-trace-jwt"
+    ]
+  },
   {
     "chain": "amoy",
     "client": "bor",