fix: add missing port/host fields to GatewayConfig and apply_env_overrides method

- Add port and host fields to GatewayConfig struct
- Add default_gateway_port() and default_gateway_host() functions
- Add apply_env_overrides() method to Config for env var support
- Fix test to include new GatewayConfig fields

All tests pass.

src/providers/gemini.rs

@@ -95,7 +95,7 @@ struct GeminiCliSettings {
 
 impl GeminiProvider {
     /// Create a new Gemini provider.
-    /// 
+    ///
     /// Authentication priority:
     /// 1. Explicit API key passed in
     /// 2. `GEMINI_API_KEY` environment variable