Refactor email status clients to support rate-limiter options

[maleksal]

Overview

Currently, ZerobounceClient and MailercheckClient do not support configurable rate-limiter options. This makes it difficult to control API request rates and properly test rate-limiting behavior.

Tasks

• Ensure both clients have configurable rate-limiter options via constructor.
• Use environment variables for rate-limiter settings.
• Add unit tests to validate rate-limiter functionality.

Guidelines

• Create a separate PR for each sub-issue.
• In the PR description, reference the issue to close it automatically when your PR is merged. Read more:

  Resolves #<issue_number>