Description

1. The `MongoWriteConcernError` constructor currently overwrites the top-level error code (MongoWriteConcernError.code) with the nested (MongoWriteConcernError.writeConcernError). If there was originally no top-level code, then the nested writeConcernError's code will be set as the top-level code.
2.  When determining whether or not to add `retryableWriteLabel`, `MongoWriteConcernError's` result property is incorrectly accessed  (it should be `MongoWriteConcernError.result.writeConcernError.code`)

Unknowns

• How do the changes in this commit affect the behavior of MongoWriteConcernError?

Acceptance Criteria

Implementation Requirements

• Ensure that MongoWriteConcernError is correctly formed such that the original top-level code is preserved
  • If no top-level code exists, MongoWriteConcernError.code should be set to the nested code
  • If a top-level code is passed into the constructor, it shouldn't be changed or overwritten by the nested writeConcernError.code
• Add stricter typing for MongoWriteConcernError properties so that nested properties are not improperly accessed

Testing Requirements

• All existing tests pass
• Add unit test explicitly asserting that top-level `error.code` is not overwritten by nested `error.writeConcernError.code`