refactor(tests): reduce duplication with test helper functions

Extract common DID document patterns into helper functions:
- makeTestDIDDoc: creates basic DID document
- makeTestDIDDocWithKA: adds keyAgreement section
- makeTestVM: creates verification methods
- makeTestX25519JWK: creates test X25519 JWK
- Add constants for test DID values

This removes ~100 lines of repetitive test setup code.

Author: leifj