fix(gemini): resolve 400 stream errors and improve client compatibility

- Filter out empty text parts in Gemini requests to avoid 400 errors.
- Inject 'assistant' role into the first streaming chunk for better compatibility with clients like opencode.
- Fallback to tool_call_id for Gemini function responses when name is missing.

src/providers/gemini.rs

@@ -200,12 +200,14 @@ impl GeminiProvider {
             if msg.role == "system" {
                 for part in msg.content {
                     if let ContentPart::Text { text } = part {
-                        system_parts.push(GeminiPart {
-                            text: Some(text),
-                            inline_data: None,
-                            function_call: None,
-                            function_response: None,
-                        });
+                        if !text.trim().is_empty() {
+                            system_parts.push(GeminiPart {
+                                text: Some(text),
+                                inline_data: None,
+                                function_call: None,
+                                function_response: None,
+                            });
+                        }
                     }
                 }
                 continue;
@@ -230,7 +232,8 @@ impl GeminiProvider {
                     })
                     .unwrap_or_default();
 
-                let name = msg.name.clone().unwrap_or_default();
+                // Gemini function response MUST have a name. Fallback to tool_call_id if name is missing.
+                let name = msg.name.clone().or_else(|| msg.tool_call_id.clone()).unwrap_or_else(|| "unknown_function".to_string());
                 let response_value = serde_json::from_str::<Value>(&text_content)
                     .unwrap_or_else(|_| serde_json::json!({ "result": text_content }));
 
@@ -249,7 +252,7 @@ impl GeminiProvider {
                     // Include text content if present
                     for p in &msg.content {
                         if let ContentPart::Text { text } = p {
-                            if !text.is_empty() {
+                            if !text.trim().is_empty() {
                                 parts.push(GeminiPart {
                                     text: Some(text.clone()),
                                     inline_data: None,
@@ -279,12 +282,14 @@ impl GeminiProvider {
                 for part in msg.content {
                     match part {
                         ContentPart::Text { text } => {
-                            parts.push(GeminiPart {
-                                text: Some(text),
-                                inline_data: None,
-                                function_call: None,
-                                function_response: None,
-                            });
+                            if !text.trim().is_empty() {
+                                parts.push(GeminiPart {
+                                    text: Some(text),
+                                    inline_data: None,
+                                    function_call: None,
+                                    function_response: None,
+                                });
+                            }
                         }
                         ContentPart::Image(image_input) => {
                             let (base64_data, mime_type) = image_input

src/server/mod.rs

@@ -250,10 +250,18 @@ async fn chat_completions(
                 // Build stream that yields events wrapped in Result
                 let stream = async_stream::stream! {
                     let mut aggregator = Box::pin(aggregating_stream);
+                    let mut first_chunk = true;
                     
                     while let Some(chunk_result) = aggregator.next().await {
                         match chunk_result {
                             Ok(chunk) => {
+                                let role = if first_chunk {
+                                    first_chunk = false;
+                                    Some("assistant".to_string())
+                                } else {
+                                    None
+                                };
+
                                 let response = ChatCompletionStreamResponse {
                                     id: stream_id_sse.clone(),
                                     object: "chat.completion.chunk".to_string(),
@@ -262,7 +270,7 @@ async fn chat_completions(
                                     choices: vec![ChatStreamChoice {
                                         index: 0,
                                         delta: ChatStreamDelta {
-                                            role: None,
+                                            role,
                                             content: Some(chunk.content),
                                             reasoning_content: chunk.reasoning_content,
                                             tool_calls: chunk.tool_calls,