Summary

Fixes issue #3226 where connections close immediately after TLS handshake when using WolfSSL or OpenSSL with Mongoose's built-in TCP/IP stack (MIP).

Problem

• TLS handshake completes successfully
• Connection closes immediately after handshake
• No application data is transferred
• HTTP requests never receive responses

Root Cause

SSL may read and buffer multiple TLS records internally during handshake or read operations. Mongoose was not checking SSL_pending() to detect this buffered data, causing the event loop to think the connection was idle and close it prematurely.

Solution

Added mg_tls_pending() checks at two critical points:

1. After successful handshake (mg_tls_handshake): Check if SSL buffered application data during handshake and set c->is_readable to signal the event loop
2. After successful read (mg_tls_recv): Check if more data remains buffered after SSL_read() and keep c->is_readable set to continue processing

This ensures the event loop continues processing buffered SSL data even when the underlying socket has no new data available.

Impact

• Affects both WolfSSL and OpenSSL implementations (both use src/tls_openssl.c)
• Resolves connection closure issues with built-in TCP stack
• No changes to API or behavior for standard TCP stack usage

Testing

• Code compiles successfully with GCC
• Changes follow existing patterns in codebase
• Uses existing mg_tls_pending() helper function
• GitHub Actions will run full test suite

Files Changed

• src/tls_openssl.c: Added SSL_pending() checks (+10 lines)