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>

test/test.cc

@@ -14626,12 +14626,6 @@ TEST_F(SSLOpenStreamTest, PostChunked) {
 // SSL peer sends a close_notify after the body, the client must treat it as a
 // clean EOF and return a successful response rather than an error.
 TEST(SSLTest, ResponseBodyTerminatedByConnectionClose) {
-#ifdef CPPHTTPLIB_MBEDTLS_SUPPORT
-  // TODO: mbedTLS reports a clean close_notify mid-response as a read error.
-  // Treat the EOF as a successful body terminator the way the OpenSSL/wolfSSL
-  // backends already do.
-  GTEST_SKIP() << "mbedTLS backend treats close_notify mid-response as error";
-#endif
   SSLServer svr(SERVER_CERT_FILE, SERVER_PRIVATE_KEY_FILE);
   ASSERT_TRUE(svr.is_valid());