Add unit tests and regress tests

[yosuke-wolfssl]

This PR adds the new tests to harden wolfSSH.

• Add a unit test that sets ssh->isKeying |= WOLFSSH_SELF_IS_KEYING and then calls DoNewKeys, asserting it returns WS_INVALID_STATE_E for DoNewKeys.
• Add a test that opens one session channel successfully, then attempts to open a second session channel and asserts it receives OPEN_ADMINISTRATIVELY_PROHIBITED failure for DoChannelOpen
• Add a regression test that calls DoKexInit and then injects a second SSH_MSG_KEXINIT payload, asserting that the result is WS_INVALID_STATE_E.

Addressed by f_2876, f_ 2877 and f_2878.

[Copilot]

Pull request overview

This PR adds regression coverage for key-exchange state guards and session-channel rejection behavior in the wolfSSH test suite.

Changes:

• Adds parsing helpers for channel-open failure responses.
• Adds a regression test ensuring a second session channel is rejected with OPEN_ADMINISTRATIVELY_PROHIBITED.
• Adds tests for DoNewKeys and DoKexInit invalid keying-state handling.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[wolfSSL-Fenrir-bot]

Fenrir Automated Review — PR #990

No scan targets match the changed files in this PR. Review skipped.