Custom model names can include the @ symbol by itself.

To specify the model's provider, append it after the model name using `@` as before.

This format supports cases like `google vertex ai` with a model name like `claude-3-5-sonnet@20240620`.

For instance, `claude-3-5-sonnet@20240620@vertex-ai` will be split by `split(/@(?!.*@)/)` into:

`[ 'claude-3-5-sonnet@20240620', 'vertex-ai' ]`, where the former is the model name and the latter is the custom provider.

app/components/chat.tsx

@@ -645,7 +645,7 @@ export function ChatActions(props: {
           onClose={() => setShowModelSelector(false)}
           onSelection={(s) => {
             if (s.length === 0) return;
-            const [model, providerName] = s[0].split("@");
+            const [model, providerName] = s[0].split(/@(?!.*@)/);
             chatStore.updateCurrentSession((session) => {
               session.mask.modelConfig.model = model as ModelType;
               session.mask.modelConfig.providerName =