test(NODE-7165): migrate integration/bson-decimal128 tests

[tadjik1]

Description

Summary of Changes

This PR migrates the integration tests for the Decimal128 class. The changes include:

• Converting the test files from .js to .ts.
• Replacing callbacks with async/await.
• Using this.configuration to instantiate the client.
• Importing directly from src to avoid using the legacy wrapper.

Notes for Reviewers

To simplify the review process, this PR is split into two commits:

• The first commit covers the .js to .ts conversion.
• The second commit contains the remaining refactoring changes.

What is the motivation for this change?

This work is part of a larger, ongoing initiative to convert all tests to use async/await, with the ultimate goal of removing the legacy driver wrapper.

Double check the following

• Lint is passing (npm run check:lint)
• 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

[baileympearson]

CI failures are unrelated.