feat(ui): add TrustedRouter to the Add Model provider fields

test_every_backend_provider_is_listed_in_add_model_or_frozen_as_unlisted
asserts every LlmProviders value is either served by
/public/providers/fields or frozen in ADD_MODEL_UNLISTED_PROVIDERS. The
test message asks for a provider_create_fields.json entry rather than an
addition to the frozen set, and the branch already adds TrustedRouter to
the dashboard dropdown, so it belongs in the listed set.

api_base is optional with a null default so the canonical URL is not
persisted into every credential record.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

litellm/proxy/public_endpoints/provider_create_fields.json

@@ -4004,5 +4004,33 @@
       }
     ],
     "default_model_placeholder": "cursor/claude-4-sonnet"
+  },
+  {
+    "provider": "Trustedrouter",
+    "provider_display_name": "TrustedRouter",
+    "litellm_provider": "trustedrouter",
+    "credential_fields": [
+      {
+        "key": "api_key",
+        "label": "TrustedRouter API Key",
+        "placeholder": "Your TrustedRouter API key from trustedrouter.com",
+        "tooltip": "Get your API key from the TrustedRouter dashboard at trustedrouter.com",
+        "required": true,
+        "field_type": "password",
+        "options": null,
+        "default_value": null
+      },
+      {
+        "key": "api_base",
+        "label": "API Base",
+        "placeholder": "https://api.trustedrouter.com/v1",
+        "tooltip": "TrustedRouter API base URL (defaults to https://api.trustedrouter.com/v1)",
+        "required": false,
+        "field_type": "text",
+        "options": null,
+        "default_value": null
+      }
+    ],
+    "default_model_placeholder": "trustedrouter/auto"
   }
 ]