fix: expand gateway supported providers based on Vercel AI Gateway docs

Add xai and bedrock to supported providers list. The Vercel AI Gateway
supports OpenAI, Anthropic, Google, xAI, Amazon Bedrock, and others.

Excluded providers:
- ollama: Local-only, not routable through cloud gateway
- openrouter: Already a gateway/aggregator

Author: ammar-agent

src/browser/hooks/useGatewayModels.ts

@@ -7,9 +7,15 @@ const GATEWAY_AVAILABLE_KEY = "gateway-available";
 
 /**
  * Providers that Mux Gateway supports routing to.
+ * Based on Vercel AI Gateway supported providers.
  * Only models from these providers can use the gateway toggle.
+ *
+ * Excluded:
+ * - ollama: Local-only provider, not routable through cloud gateway
+ * - openrouter: Already a gateway/aggregator, routing through another gateway is redundant
+ * - mux-gateway: Already gateway format
  */
-const GATEWAY_SUPPORTED_PROVIDERS = new Set(["anthropic", "openai", "google"]);
+const GATEWAY_SUPPORTED_PROVIDERS = new Set(["anthropic", "openai", "google", "xai", "bedrock"]);
 
 /**
  * Check if a model's provider is supported by Mux Gateway.