Unify getDiscriminator to produce a single discriminator object for both
single-key and compound cases. Single-key returns standard {propertyName,
mapping}. Compound falls back to extending the object with x-propertyNames
and x-mapping for multi-property discrimination.
Simplify call sites: OpenAPI3 uses 'discriminator', Swagger2 uses
'x-discriminator' — no more split keys.
Add conditions to all 7 Algo models (AlgoArgon2, AlgoBcrypt, AlgoMd5,
AlgoPhpass, AlgoScrypt, AlgoScryptModified, AlgoSha) to enable
discriminator generation for hashOptions unions.