Add EC public key auto derivation from private key

[aidangarske]

Description

Implements auto-derivation of EC public keys from private keys when importing private-only keys, matching OpenSSL master behavior (PR #29054, merged Dec 15, 2025). Failure caught here with master ossl build. Needed for #337

Changes

• Implementation (src/wp_ecc_kmgmt.c): Auto-derive public key from private key when:
  • OpenSSL version > 3.6.0 (master or future releases)
  • Private key is present but public key is missing
  • Curve is set
• Tests (test/test_ecc.c):
  • Updated test_ec_import_priv() to verify auto-derivation behavior based on OpenSSL version
  • Updated test_ec_auto_derive_pubkey() to only run when OpenSSL > 3.6.0
  • Added new unit test test_ec_auto_derive_pubkey() to specifically verify auto-derivation functionality
  • Wrapped test_ec_auto_derive_pub() in version guard to avoid unused function warnings

Compatibility

• OpenSSL ≤ 3.6.0: No auto-derivation (backward compatible)
• OpenSSL > 3.6.0 (master/future): Auto-derivation enabled to match OpenSSL behavior
• verified behavior doesn't change with all master CI and all openssl-3.6.0 CI

[padelsbach]

Looks great, thanks