KEX Fix

[ejohnstown]

1. When disabling mlkem768nistp256-sha256, there's a crash when doing a mlkem768x25519-sha256 KEX. It tries to do a DH key exchange instead of x25519 with ML-KEM. Modified the check.
2. Fixed a guard where the build was treating not having mlkem768nistp256-sha256 like a FIPS build disallowing using ML-KEM when generating sesssion keys. It shouldn't make that check.
3. Added a test case to the KEM test.

[Copilot]

Pull request overview

This PR fixes two bugs related to ML-KEM key exchange (KEX) handling in wolfSSH and adds a corresponding test case.

Changes:

• Fixed a crash when performing mlkem768x25519-sha256 KEX while mlkem768nistp256-sha256 is disabled, by replacing a complex negated condition with a direct useDh flag check.
• Removed an incorrect guard in GenerateKey that was treating the absence of WOLFSSH_NO_NISTP256_MLKEM768_SHA256 like a FIPS build, preventing ML-KEM session key generation.
• Added a test case for the mlkem768nistp256-sha256 KEX algorithm.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/internal.c	Fixes the flawed DH condition check and the incorrect GenerateKey preprocessor guard
tests/kex.c	Adds a test case for mlkem768nistp256-sha256 KEX

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