Fix blocking file I/O in Response.stream_file()

The async generator was using synchronous open() and read() which
blocks the event loop. Switched to anyio.open_file() for proper
async file I/O.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

responder/models.py

@@ -464,9 +464,11 @@ class Response:
             self.mimetype = guessed or "application/octet-stream"
 
         async def file_generator():
-            with open(path, "rb") as f:
+            import anyio
+
+            async with await anyio.open_file(path, "rb") as f:
                 while True:
-                    chunk = f.read(chunk_size)
+                    chunk = await f.read(chunk_size)
                     if not chunk:
                         break
                     yield chunk