feat: add gemini-3.1-pro-preview to model lists

Add the newly released Gemini 3.1 Pro model to both the
native API fallback list and the admin UI test model dropdown.

backend/internal/pkg/gemini/models.go

@@ -21,6 +21,7 @@ func DefaultModels() []Model {
 		{Name: "models/gemini-2.5-pro", SupportedGenerationMethods: methods},
 		{Name: "models/gemini-3-flash-preview", SupportedGenerationMethods: methods},
 		{Name: "models/gemini-3-pro-preview", SupportedGenerationMethods: methods},
+		{Name: "models/gemini-3.1-pro-preview", SupportedGenerationMethods: methods},
 	}
 }
 

backend/internal/pkg/geminicli/models.go

@@ -16,6 +16,7 @@ var DefaultModels = []Model{
 	{ID: "gemini-2.5-pro", Type: "model", DisplayName: "Gemini 2.5 Pro", CreatedAt: ""},
 	{ID: "gemini-3-flash-preview", Type: "model", DisplayName: "Gemini 3 Flash Preview", CreatedAt: ""},
 	{ID: "gemini-3-pro-preview", Type: "model", DisplayName: "Gemini 3 Pro Preview", CreatedAt: ""},
+	{ID: "gemini-3.1-pro-preview", Type: "model", DisplayName: "Gemini 3.1 Pro Preview", CreatedAt: ""},
 }
 
 // DefaultTestModel is the default model to preselect in test flows.