Adds 11 new tests covering:
- Roundtrip of DiscardOldest/SubscriptionPriority/SubscriptionDisplayName/TimestampsToReturn
- Roundtrip of UserIdentity sub-object across all three TokenTypes
- Roundtrip of Deadband sub-object
- ToFlatDict/FromFlatDict for UserIdentity.* and Deadband.* dotted keys
- Validator rules: empty SubscriptionDisplayName, UsernamePassword w/o Username,
X509 w/o CertificatePath, Deadband Value <= 0, prefix propagation
Build passes; tests fail because serializer/validator have not been extended yet
(TDD red phase). Task B2 will implement the changes to drive them green.
91450ec390