NBMsgPack001: Apply [Key] consistently across members

KeyAttribute should either not be applied to any members or should be applied to all serialized members.

Data-contract attributes

The analyzer follows PolyType's member-selection rules when attributes from System.Runtime.Serialization are applied:

• On a type annotated with [DataContract], only fields and properties annotated with [DataMember] are included by default, including non-public members. Unannotated members do not require [Key].
• Without [DataContract] on the declaring type, [IgnoreDataMember] excludes a member. [DataMember] alone does not opt a non-public member into serialization.
• These rules apply separately to each member's declaring type, including base types. Applying [DataContract] to a derived type does not change which members of its base type are included. Key usage must still be consistent across all serialized members in the hierarchy.
• [PropertyShape] takes precedence over the data-contract attributes: it explicitly includes a member unless Ignore = true, which excludes it. Within a [DataContract] type, [DataMember] includes a member even if it also has [IgnoreDataMember].

Apply [Key] consistently to the members that are actually serialized, not to excluded members. [DataMember(Order = ...)] does not substitute for [Key].

Resolution

There are two fixes for this condition:

• Remove the attribute from all members. This will cause the type to serialize as a map of property name=value pairs.
• Consistently apply the attribute to all serialized members. This will cause the type to serialize as an array of values.