Dtls13: Fix handshake hangs on WANT_WRITE I/O error

[rizlik]

Description

The internal buffer index pointer wasn't incremented in case of error inside _Dtls13HandshakeRecv, causing an hangs.

Fixes zd#20686

Added tests to increase coverage from non-blocking I/O.

[Copilot]

Pull Request Overview

This PR fixes a handshake hang issue in DTLS 1.3 that occurred when a WANT_WRITE I/O error was encountered. The core fix ensures the internal buffer index pointer is updated before error handling, preventing infinite loops. Additionally, the PR corrects bugs in test infrastructure where client/server force-want-write flags were swapped, and adds comprehensive test coverage for non-blocking I/O scenarios.

Key Changes:

• Moved buffer index update in _Dtls13HandshakeRecv to occur before error checking
• Fixed swapped client/server forceWantWrite references in test I/O callbacks
• Added two new test cases to verify proper handling of WANT_WRITE conditions during DTLS 1.3 handshakes

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/dtls13.c	Fixes buffer index update timing to prevent handshake hangs on WANT_WRITE errors
tests/utils.c	Corrects swapped client/server force-want-write flag assignments in I/O callback
tests/api/test_dtls.h	Declares two new test functions for DTLS 1.3 WANT_WRITE scenarios
tests/api/test_dtls.c	Implements comprehensive tests for HRR and every-write WANT_WRITE cases
tests/api.c	Fixes incorrect side parameter in test_memio_simulate_want_write calls

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[devin-ai-integration]

🛟 Devin Lifeguard found 1 likely issues in this PR

• pointer-null-check snippet: Guard the write with a null-check, e.g., if (processedSize != NULL) *processedSize = idx;, ensuring no dereference occurs when the pointer is NULL.

@rizlik
please take a look at the above issues which Devin flagged. Devin will not fix these issues automatically.

[rizlik]

retest this please

[rizlik]

retest this please
SSL_accept error -501, can't match cipher suite