chore: update model manifest (#1616)

Automated model discovery run.

This PR updates `components/manifests/base/core/models.json` based on
probing Vertex AI endpoints. Validation passed — safe to auto-merge.

Unleash flags are synced automatically on deploy by the
`sync-model-flags` Job.
<details><summary>Diff</summary>

```diff
diff --git a/components/manifests/base/core/models.json b/components/manifests/base/core/models.json
index 605b7d5..7474bcc 100755
--- a/components/manifests/base/core/models.json
+++ b/components/manifests/base/core/models.json
@@ -157,6 +157,30 @@
       "provider": "google",
       "available": false,
       "featureGated": true
+    },
+    {
+      "id": "claude-opus-4-8",
+      "label": "Claude Opus 4.8",
+      "vertexId": "claude-opus-4-8@default",
+      "provider": "anthropic",
+      "available": false,
+      "featureGated": true
+    },
+    {
+      "id": "gemini-3-pro-image",
+      "label": "Gemini 3 Pro Image",
+      "vertexId": "gemini-3-pro-image@default",
+      "provider": "google",
+      "available": false,
+      "featureGated": true
+    },
+    {
+      "id": "gemini-3.1-flash-image",
+      "label": "Gemini 3.1 Flash Image",
+      "vertexId": "gemini-3.1-flash-image@default",
+      "provider": "google",
+      "available": false,
+      "featureGated": true
     }
   ]
 }
```
</details>

Co-authored-by: Gkrumbach07 <12587674+Gkrumbach07@users.noreply.github.com>

Author: github-actions[bot]

components/manifests/base/core/models.json

@@ -157,6 +157,30 @@
       "provider": "google",
       "available": false,
       "featureGated": true
+    },
+    {
+      "id": "claude-opus-4-8",
+      "label": "Claude Opus 4.8",
+      "vertexId": "claude-opus-4-8@default",
+      "provider": "anthropic",
+      "available": false,
+      "featureGated": true
+    },
+    {
+      "id": "gemini-3-pro-image",
+      "label": "Gemini 3 Pro Image",
+      "vertexId": "gemini-3-pro-image@default",
+      "provider": "google",
+      "available": false,
+      "featureGated": true
+    },
+    {
+      "id": "gemini-3.1-flash-image",
+      "label": "Gemini 3.1 Flash Image",
+      "vertexId": "gemini-3.1-flash-image@default",
+      "provider": "google",
+      "available": false,
+      "featureGated": true
     }
   ]
 }