Add Salesforce Billing integration

[Smartdevs17]

Overview

Implement the capability described below to improve the project.

Requirements

• Add a complete, well-tested implementation of Add Salesforce Billing integration.
• Follow existing project conventions and architecture.
• Include unit tests covering success and failure paths.
• Ensure linting and type checks pass.

Acceptance Criteria

• Feature works end-to-end and is documented.
• Tests are added and passing.
• No regressions in existing functionality.

Additional Context

Refer to the project README and existing code patterns for guidance. This issue is actionable by any contributor.