Test ML-KEM mixed-NULL KEM requests

[aidangarske]

The provider already implements OpenSSL-compatible mixed-NULL handling in e44edc6.

Add EVP_PKEY encapsulate/decapsulate regression coverage to ensure mixed-NULL requests are rejected while valid size queries remain supported.

[Copilot]

🟡 Not ready to approve

The new test should better align with existing ML-KEM test patterns (coverage across levels and clearer diagnostics/comment intent) to reliably meet the stated regression goals.

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Pull request overview

Adds a new unit/regression test to validate OpenSSL-compatible handling of ML-KEM “mixed-NULL” KEM API usage, ensuring invalid mixed-NULL encapsulation output requests are rejected (and integrating that test into the unit test runner).

Changes:

• Registers a new ML-KEM unit test (test_mlkem_mixed_null) in the unit test declarations and test-case list.
• Adds test_mlkem_mixed_null() to exercise mixed-NULL EVP_PKEY_encapsulate() calls (and a NULL-ciphertext EVP_PKEY_decapsulate() call).

File summaries

File	Description
test/unit.h	Declares the new ML-KEM mixed-NULL test function.
test/unit.c	Adds the new ML-KEM mixed-NULL test to the unit test execution list.
test/test_mlkem.c	Implements the new regression test for mixed-NULL encapsulate/decapsulate calls.

Review details

Suppressed comments (2)

test/test_mlkem.c:364

• Same as above: if mixed-NULL encapsulate (secret=NULL) unexpectedly succeeds, the test should emit a clear error message to simplify debugging.

    if (err == 0) {
        ctLen = sizeof(ct);
        secretLen = sizeof(secret);
        err = EVP_PKEY_encapsulate(ectx, ct, &ctLen, NULL,
            &secretLen) == 1;
    }

test/test_mlkem.c:372

• If decapsulate unexpectedly succeeds with a NULL ciphertext pointer, adding a PRINT_ERR_MSG would make the failure clearer (consistent with other negative-path assertions in this file).

    if (err == 0) {
        err = EVP_PKEY_decapsulate(dctx, secret, &secretLen, NULL,
            ctLen) == 1;
    }

• Files reviewed: 3/3 changed files
• Comments generated: 3
• Review effort level: Lite

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[padelsbach]

looks good, will merge when CI wraps up