fix(identity-wallet): rewrite plc_monitor behavior tests with real crypto keys
The previous behavior tests used fake did:key URIs that don't correspond
to real P-256 keys, causing verify_plc_operation to always fail. Tests
also missed required IdentityStore setup (add_identity + device key
generation), causing IdentityNotFound errors.
Fixed by:
- Adding setup_identity() helper that registers DIDs, generates device
keys via IdentityStore, and retrieves private key bytes from Keychain
- Using crypto::build_did_plc_genesis_op with real device key material
- Using crypto::generate_p256_keypair for "other" keys in unauthorized tests
- Unique DID names per test to avoid keychain state interference
- Running tests with --test-threads=1 for keychain isolation
All 15 tests pass (8 serialization + 7 behavior covering AC6.1, AC6.2,
AC6.3, AC6.7, AC6.8, and multi-identity variants).