Fix line splitting in format_sse_event to comply with SSE spec

Resolves issue where string.splitlines() incorrectly splits on non-LF/CRLF characters, which broke the Server-Sent Events stream specification.

fastapi/sse.py

@@ -206,14 +206,14 @@ def format_sse_event(
     lines: list[str] = []
 
     if comment is not None:
-        for line in comment.splitlines():
+        for line in comment.replace("\r\n", "\n").replace("\r", "\n").split("\n"):
             lines.append(f": {line}")
 
     if event is not None:
         lines.append(f"event: {event}")
 
     if data_str is not None:
-        for line in data_str.splitlines():
+        for line in data_str.replace("\r\n", "\n").replace("\r", "\n").split("\n"):
             lines.append(f"data: {line}")
 
     if id is not None: