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/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,