fix(gemini): implement dynamic API versioning and support Gemini 3

- Switch to v1beta endpoint for 'preview' and 'thinking' models.
- Update model version checks to include gemini-3 as a known version.
- Use get_base_url helper to construct dynamic URLs for both streaming and non-streaming requests.

src/providers/gemini.rs

@@ -439,6 +439,17 @@ impl GeminiProvider {
     }
 }
 
+    /// Determine the appropriate base URL for the model.
+    /// "preview" models often require the v1beta endpoint.
+    fn get_base_url(&self, model: &str) -> String {
+        if model.contains("preview") || model.contains("thinking") {
+            self.config.base_url.replace("/v1", "/v1beta")
+        } else {
+            self.config.base_url.clone()
+        }
+    }
+}
+
 #[async_trait]
 impl super::Provider for GeminiProvider {
     fn name(&self) -> &str {
@@ -456,10 +467,16 @@ impl super::Provider for GeminiProvider {
     async fn chat_completion(&self, request: UnifiedRequest) -> Result<ProviderResponse, AppError> {
         let mut model = request.model.clone();
         
-        // Normalize model name: If it's a known Gemini model, use it;
+        // Normalize model name: If it's a known Gemini model version, use it;
-        // otherwise, if it starts with gemini- but is unknown (e.g. gemini-3-flash-preview), 
+        // otherwise, if it starts with gemini- but is an unknown legacy version, 
         // fallback to the default model to avoid 400 errors.
-        if !model.starts_with("gemini-1.5") && !model.starts_with("gemini-2.0") && model.starts_with("gemini-") {
+        // We now allow gemini-3+ as valid versions.
+        let is_known_version = model.starts_with("gemini-1.5") || 
+                              model.starts_with("gemini-2.0") || 
+                              model.starts_with("gemini-2.5") ||
+                              model.starts_with("gemini-3");
+
+        if !is_known_version && model.starts_with("gemini-") {
             tracing::info!("Mapping unknown Gemini model {} to default {}", model, self.config.default_model);
             model = self.config.default_model.clone();
         }
@@ -475,6 +492,7 @@ impl super::Provider for GeminiProvider {
         let generation_config = if request.temperature.is_some() || request.max_tokens.is_some() {
             // Some Gemini models (especially 1.5) have lower max_output_tokens limits (e.g. 8192)
             // than what clients like opencode might request. Clamp to a safe maximum.
+            // Note: Gemini 2.0+ supports much higher limits, but 8192 is a safe universal floor.
             let max_tokens = request.max_tokens.map(|t| t.min(8192));
             
             Some(GeminiGenerationConfig {
@@ -493,7 +511,8 @@ impl super::Provider for GeminiProvider {
             tool_config,
         };
 
-        let url = format!("{}/models/{}:generateContent", self.config.base_url, model);
+        let base_url = self.get_base_url(&model);
+        let url = format!("{}/models/{}:generateContent", base_url, model);
 
         let response = self
             .client
@@ -595,7 +614,12 @@ impl super::Provider for GeminiProvider {
         let mut model = request.model.clone();
         
         // Normalize model name: fallback to default if unknown Gemini model is requested
-        if !model.starts_with("gemini-1.5") && !model.starts_with("gemini-2.0") && model.starts_with("gemini-") {
+        let is_known_version = model.starts_with("gemini-1.5") || 
+                              model.starts_with("gemini-2.0") || 
+                              model.starts_with("gemini-2.5") ||
+                              model.starts_with("gemini-3");
+
+        if !is_known_version && model.starts_with("gemini-") {
             tracing::info!("Mapping unknown Gemini model {} to default {}", model, self.config.default_model);
             model = self.config.default_model.clone();
         }
@@ -629,9 +653,10 @@ impl super::Provider for GeminiProvider {
             tool_config,
         };
 
+        let base_url = self.get_base_url(&model);
         let url = format!(
             "{}/models/{}:streamGenerateContent?alt=sse",
-            self.config.base_url, model,
+            base_url, model,
         );
 
         // (no fallback_request needed here)
@@ -646,7 +671,7 @@ impl super::Provider for GeminiProvider {
         // Prepare clones for HTTP fallback usage inside non-streaming paths.
         let http_client = self.client.clone();
         let http_api_key = self.api_key.clone();
-        let http_base = self.config.base_url.clone();
+        let http_base = base_url.clone();
         let gemini_request_clone = gemini_request.clone();
 
         let es_result = reqwest_eventsource::EventSource::new(