Fix mbedTLS close_notify mid-response handling

The mbedTLS backend's read() returned -1 with err.code = PeerClosed when
the peer sent close_notify, while OpenSSL and wolfSSL surface it as 0
(clean EOF). The result was that an SSL response without Content-Length
or chunked Transfer-Encoding — terminated by connection close — was
reported as "Failed to read connection" on mbedTLS, even though the
body had been fully delivered.

Translate PeerClosed into a return value of 0 to match the other
backends. This re-enables SSLTest.ResponseBodyTerminatedByConnectionClose
on mbedTLS.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

httplib.h

@@ -17956,6 +17956,9 @@ inline ssize_t read(session_t session, void *buf, size_t len, TlsError &err) {
   err.code = impl::map_mbedtls_error(ret, err.sys_errno);
   err.backend_code = static_cast<uint64_t>(-ret);
   impl::mbedtls_last_error() = ret;
+  // mbedTLS signals a clean close_notify via a negative error code rather
+  // than 0; surface it as a clean EOF the way OpenSSL/wolfSSL do.
+  if (err.code == ErrorCode::PeerClosed) { return 0; }
   return -1;
 }