fix: ML-KEM key size test expectations wrong (expects 2400, gets 64 bytes)

[CodeMonkeyCybersecurity]

Summary

Tests in pkg/crypto/pq/mlkem_test.go expect ML-KEM-768 private key size of 2400 bytes but get 64 bytes. This causes test failures across TestGenerateMLKEMKeypair, TestValidateMLKEMPrivateKey, and TestGenerateHybridKeypair.

Evidence

expected: 2400
actual  : 64

Root Cause

Likely a Go library version change where the key representation format changed, or the key generation function returns a seed rather than the expanded key.

Impact

• pkg/crypto test suite fails
• Coverage measurements are inaccurate

Acceptance Criteria

• All ML-KEM tests pass
• Key size assertions match actual library output
• Coverage for pkg/crypto/pq exceeds 70%