feat(mistral): add mistral-medium-3-5 model

Add mistral-medium-3-5 to model catalog with input cost 1.5 and output cost 7.5.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

extensions/mistral/openclaw.plugin.json

@@ -90,6 +90,19 @@
               "cacheWrite": 0
             }
           },
+          {
+            "id": "mistral-medium-3-5",
+            "name": "Mistral Medium 3.5",
+            "input": ["text", "image"],
+            "contextWindow": 262144,
+            "maxTokens": 8192,
+            "cost": {
+              "input": 1.5,
+              "output": 7.5,
+              "cacheRead": 0,
+              "cacheWrite": 0
+            }
+          },
           {
             "id": "mistral-small-latest",
             "name": "Mistral Small (latest)",