The `System` namespace was added to `EnvelopeStatus.cs` to enable additional functionality. A documentation URL in the comments was updated to point to a new location, replacing the outdated link.
Introduced a new static class `EnvelopeStatusExtensions` with two extension methods for the `EnvelopeStatus` enum:
- `IsActive`: Checks if the status is active (between `EnvelopeCreated` and `EnvelopePartlySigned`).
- `IsCompleted`: Checks if the status is completed (between `EnvelopeCompletelySigned` and `EnvelopeWithdrawn`).