Harden tool endpoint origin validation and source fingerprinting

docs/contracts/agent-builder-mcp/examples/agentbuilder-v0.3/tools-call-execution-error-response.json

@@ -5,7 +5,7 @@
     "content": [
       {
         "type": "text",
-        "text": "customer.search@1.0.0: timed out"
+        "text": "{\"reasonCode\":\"TOOL_TIMEOUT\",\"message\":\"Tool execution timed out.\"}"
       }
     ],
     "isError": true

docs/contracts/agent-builder-mcp/examples/agentbuilder-v0.3/tools-call-invalid-params-response.json

@@ -3,6 +3,10 @@
   "id": 3,
   "error": {
     "code": -32602,
-    "message": "Invalid params: 'query' is required"
+    "message": "Invalid params",
+    "data": {
+      "reasonCode": "INVALID_PARAMS",
+      "message": "Request parameters are invalid."
+    }
   }
 }