Add AccountKey tests

[toddr-bot]

What

Comprehensive test coverage for Net::ACME2::AccountKey, the crypto backend abstraction layer that had zero dedicated tests.

Why

AccountKey is on the critical path for every ACME operation — it handles key parsing, backend selection (CryptX vs Crypt::Perl), signing, and JWK export. All existing tests only exercised it indirectly through JWTMaker or the mock ACME server.

How

39 tests covering:

• Constructor with RSA and ECDSA keys (both PEM and DER formats)
• Key type detection via PEM header guessing and Crypt::Perl fallback
• Public JWK export (correct structure, no private key leakage)
• JWK thumbprint determinism and uniqueness
• JWA algorithm selection for P-256 and P-384 curves
• RS256 signing (determinism) and JWA signing (non-determinism)
• Signature verification round-trip via Crypt::Perl
• $@ preservation across constructor (CryptX eval guards)
• PEM vs DER equivalence for JWK and thumbprint output

Testing

make test — all 346 tests pass (21 files including this new one).

🤖 Generated with Claude Code

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Quality Report

Changes: 2 files changed, 279 insertions(+)

Code scan: clean

Tests: passed (OK)

Branch hygiene: clean

Generated by Kōan post-mission quality pipeline