[bitreq] Use saturating_add for chunked content-length accumulation

Use saturating_add instead of += when accumulating content_length in
chunked transfer encoding. This prevents integer overflow on 32-bit
systems where a malicious server could send chunk sizes that cause
the accumulated length to wrap around.

Co-Authored-By: Claude AI

Author: tnull

bitreq/src/response.rs

@@ -576,7 +576,7 @@ macro_rules! define_read_methods {
                     return None;
                 }
                 *chunk_length = incoming_length;
-                *content_length += incoming_length;
+                *content_length = content_length.saturating_add(incoming_length);
             }
 
             if *chunk_length > 0 {