feat: add SSE streaming, thinking models, and modernized Part struct

examples/text-from-text-streaming.rs

@@ -23,11 +23,11 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
     let request = GenerateContentRequest::builder().contents(prompt).build();
 
     let mut queue = gemini
-        .generate_content_stream(&request, "gemini-2.0-flash-001")
+        .stream_generate_content(&request, "gemini-2.0-flash-001")
         .await?;
 
-    while let Some(Ok(response)) = queue.next().await {
-        println!("Response: {:?}", response);
+    while let Some(response) = queue.next().await {
+        println!("Response: {:?}", response?);
     }
 
     Ok(())