fix: handle UTF-8 multi-byte characters split across stream chunk boundaries (#1923)

* fix: handle UTF-8 multi-byte characters split across stream chunk boundaries

Replace String::from_utf8_lossy with append_utf8_safe in all four SSE
streaming paths. When a multi-byte UTF-8 character (e.g. Chinese, emoji)
is split across TCP chunk boundaries, from_utf8_lossy silently replaces
the incomplete halves with U+FFFD (�). This caused intermittent garbled
output in Claude Code when using the Copilot reverse proxy, because the
format conversion streams reconstruct SSE events from the corrupted buffer.

The new append_utf8_safe function preserves incomplete trailing bytes in
a remainder buffer and merges them with the next chunk before decoding,
ensuring characters are never split during UTF-8 conversion.

Fixes: intermittent U+FFFD replacement characters in Claude Code output
via Copilot proxy (not reproducible with direct Copilot connections like
opencode because they pass through raw bytes without format conversion).

* style: fix cargo fmt formatting in UTF-8 boundary tests

---------

Co-authored-by: Cod1ng <codingts@gmail.com>
Co-authored-by: encodets <encodets@gmail.com>

src-tauri/src/proxy/response_processor.rs

@@ -568,6 +568,7 @@ pub fn create_logged_passthrough_stream(
 ) -> impl Stream<Item = Result<Bytes, std::io::Error>> + Send {
     async_stream::stream! {
         let mut buffer = String::new();
+        let mut utf8_remainder: Vec<u8> = Vec::new();
         let mut collector = usage_collector;
         let mut is_first_chunk = true;
 
@@ -619,8 +620,7 @@ pub fn create_logged_passthrough_stream(
                         );
                     }
                     is_first_chunk = false;
-                    let text = String::from_utf8_lossy(&bytes);
-                    buffer.push_str(&text);
+                    crate::proxy::sse::append_utf8_safe(&mut buffer, &mut utf8_remainder, &bytes);
 
                     // 尝试解析并记录完整的 SSE 事件
                     while let Some(pos) = buffer.find("\n\n") {