Key state consistency in PQDSA_KEY setter functions

[justsmth]

Description of changes:

The PQDSA_KEY setter functions could leave the key structure in an inconsistent state where stale fields no longer corresponded to the current key material.

• PQDSA_KEY_set_raw_private_key: Did not clear key->seed on success. If the key previously held a seed (or had an uninitialized seed buffer from PQDSA_KEY_init), pqdsa_priv_encode would serialize stale or uninitialized seed bytes, since it gates on key->seed != NULL.
• PQDSA_KEY_init: Used OPENSSL_malloc (uninitialized) for all three buffers. Changed to OPENSSL_zalloc as a defense-in-depth measure.
• PQDSA_KEY_get0_dsa: Added a NULL check on the input, consistent with other functions in this file.

Call-outs:

The primary fix is in PQDSA_KEY_set_raw_private_key. Current code paths through the public API are not affected because EVP_PKEY_pqdsa_set_params uses PQDSA_KEY_new (which zero-initializes all fields), but these functions are exposed via internal.h and the inconsistency could be triggered by internal callers.

Separately, pqdsa_priv_decode currently ignores the pubkey parameter from OneAsymmetricKey, unlike the Ed25519 implementation which validates the provided public key against the derived one. This could be addressed as a follow-up.

Testing:

All existing ML-DSA/PQDSA tests pass (76 tests).

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.51%. Comparing base (0628190) to head (c60e56a).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
crypto/fipsmodule/pqdsa/pqdsa.c	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3040      +/-   ##
==========================================
+ Coverage   78.35%   78.51%   +0.16%     
==========================================
  Files         689      689              
  Lines      121010   121015       +5     
  Branches    16992    16998       +6     
==========================================
+ Hits        94813    95019     +206     
+ Misses      25302    25097     -205     
- Partials      895      899       +4     

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