fix(gemini): resolve 404 by fixing double-beta in API URL

- Update get_base_url to only perform replacement if the base_url specifically ends with /v1.
- This prevents malformed URLs like /v1betabeta when the base_url was already configured as v1beta.

src/providers/gemini.rs

@@ -563,13 +563,16 @@ impl GeminiProvider {
     /// Determine the appropriate base URL for the model.
     /// "preview" models often require the v1beta endpoint, but newer promoted ones may be on v1.
     fn get_base_url(&self, model: &str) -> String {
-        // Gemini 3 and other reasoning/preview models often perform better or 
+        let base = &self.config.base_url;
-        // strictly require v1beta for advanced features like thought_signatures.
+        
-        if model.contains("preview") || model.contains("thinking") || model.contains("gemini-3") {
+        // If the model requires v1beta but the base is currently v1
-            self.config.base_url.replace("/v1", "/v1beta")
+        if (model.contains("preview") || model.contains("thinking") || model.contains("gemini-3")) && base.ends_with("/v1") {
-        } else {
+            return base.replace("/v1", "/v1beta");
-            self.config.base_url.clone()
         }
+        
+        // If the model is a standard model but the base is v1beta, we could downgrade it,
+        // but typically v1beta is a superset, so we just return the base as configured.
+        base.clone()
     }
 
     /// Default safety settings to avoid blocking responses.