refactor(proxy): extract take_sse_block helper with CRLF delimiter support

Replace inline `buffer.find("\n\n")` SSE splitting logic across streaming,
streaming_responses, response_handler, and response_processor with a shared
`take_sse_block` function that handles both `\n\n` and `\r\n\r\n` delimiters.

src-tauri/src/proxy/sse.rs

@@ -4,9 +4,27 @@ pub(crate) fn strip_sse_field<'a>(line: &'a str, field: &str) -> Option<&'a str>
         .or_else(|| line.strip_prefix(&format!("{field}:")))
 }
 
+#[inline]
+pub(crate) fn take_sse_block(buffer: &mut String) -> Option<String> {
+    let mut best: Option<(usize, usize)> = None;
+
+    for (delimiter, len) in [("\r\n\r\n", 4usize), ("\n\n", 2usize)] {
+        if let Some(pos) = buffer.find(delimiter) {
+            if best.is_none_or(|(best_pos, _)| pos < best_pos) {
+                best = Some((pos, len));
+            }
+        }
+    }
+
+    let (pos, len) = best?;
+    let block = buffer[..pos].to_string();
+    buffer.drain(..pos + len);
+    Some(block)
+}
+
 #[cfg(test)]
 mod tests {
-    use super::strip_sse_field;
+    use super::{strip_sse_field, take_sse_block};
 
     #[test]
     fn strip_sse_field_accepts_optional_space() {
@@ -28,4 +46,26 @@ mod tests {
         );
         assert_eq!(strip_sse_field("id:1", "data"), None);
     }
+
+    #[test]
+    fn take_sse_block_supports_lf_delimiters() {
+        let mut buffer = "data: {\"ok\":true}\n\nrest".to_string();
+
+        assert_eq!(
+            take_sse_block(&mut buffer),
+            Some("data: {\"ok\":true}".to_string())
+        );
+        assert_eq!(buffer, "rest");
+    }
+
+    #[test]
+    fn take_sse_block_supports_crlf_delimiters() {
+        let mut buffer = "data: {\"ok\":true}\r\n\r\nrest".to_string();
+
+        assert_eq!(
+            take_sse_block(&mut buffer),
+            Some("data: {\"ok\":true}".to_string())
+        );
+        assert_eq!(buffer, "rest");
+    }
 }