fix(gemini): resolve 404 by defaulting Gemini 3+ to v1 endpoint

- Update get_base_url to prefer v1 for Gemini 3.0+ models even if they contain 'preview'.
- Add tracing::debug logs for the final API URLs used in both streaming and non-streaming requests.

src/providers/gemini.rs

@@ -439,9 +439,11 @@ impl GeminiProvider {
     }
 
     /// Determine the appropriate base URL for the model.
-    /// "preview" models often require the v1beta endpoint.
+    /// "preview" models often require the v1beta endpoint, but newer promoted ones may be on v1.
     fn get_base_url(&self, model: &str) -> String {
-        if model.contains("preview") || model.contains("thinking") {
+        // Only use v1beta for older preview models or specific "thinking" experimental models.
+        // Gemini 3.0+ models are typically released on v1 even in preview.
+        if (model.contains("preview") && !model.contains("gemini-3")) || model.contains("thinking") {
             self.config.base_url.replace("/v1", "/v1beta")
         } else {
             self.config.base_url.clone()
@@ -512,6 +514,7 @@ impl super::Provider for GeminiProvider {
 
         let base_url = self.get_base_url(&model);
         let url = format!("{}/models/{}:generateContent", base_url, model);
+        tracing::debug!("Calling Gemini API: {}", url);
 
         let response = self
             .client
@@ -657,6 +660,7 @@ impl super::Provider for GeminiProvider {
             "{}/models/{}:streamGenerateContent?alt=sse",
             base_url, model,
         );
+        tracing::debug!("Calling Gemini Stream API: {}", url);
 
         // (no fallback_request needed here)