Openssl: openssl_dh_compute_key() — DH shared secret API (#6596)

[PurHur]

Summary

• Implement openssl_dh_compute_key() on VM using existing libcrypto FFI (EVP_PKEY_copy_parameters + EVP_PKEY_set1_encoded_public_key + EVP_PKEY_derive), matching php-src ext/openssl/openssl_backend_v3.c.
• Refactor VmOpensslPkeyDeriveNative::derive() to share deriveWithPeer() with the new DH path.
• Add compliance PHPT + maintainer repro with fixed DH test vectors.

php-src reference: ext/openssl/openssl_backend_v3.c — php_openssl_dh_compute_key()

Test plan

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php test/repro-maintainer/parity_openssl_dh_compute_key.php
php bin/vm.php test/repro-maintainer/parity_openssl_dh_compute_key.php
'
# Zend: ok
# VM:   ok

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && ./vendor/bin/phpunit --filter openssl_dh_compute_key'
# OK (2 tests, 2 assertions)

php script/capability-matrix.php
php script/bootstrap-inventory.php --check
./script/check-generated-docs.sh
# all green

Closes #6596

Made with Cursor