fix(NODE-6276): preserve top level error code MongoWriteConcernError

[aditi-khare-mongoDB]

Description

Preserve top level error code MongoWriteConcernError

What is changing?

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

Is there new documentation needed for these changes?

No

What is the motivation for this change?

Bug fix

Release Highlight

MongoWriteConcernError.code is overwritten by nested code within MongoWriteConcernError.result.writeConcernError.code

MongoWriteConcernError is now correctly formed such that the original top-level code is preserved

• If no top-level code exists, MongoWriteConcernError.code should be set to MongoWriteConcernError.result.writeConcernError.code
• If a top-level code is passed into the constructor, it shouldn't be changed or overwritten by the nested writeConcernError.code

Double check the following

• Ran npm run check:lint script
• Self-review completed using the steps outlined here
• PR title follows the correct format: type(NODE-xxxx)[!]: description
  • Example: feat(NODE-1234)!: rewriting everything in coffeescript
• Changes are covered by tests
• New TODOs have a related JIRA ticket