fix: document OffPayrollWorker value in EmploymentStatus enum

[mikkelgaub-planday]

The Payroll UK API already returns OffPayrollWorker as an EmploymentStatus value, but it was omitted from the enum in the spec by oversight. This adds the missing value so generated SDKs can deserialize existing GET responses.

Why this is not a breaking change

• The isOffPayrollWorker boolean flag already exists on the employee DTO and is how off-payroll workers are created; that behaviour is unchanged.
• Domain validation already rejects attempts to pass OffPayrollWorker as an EmploymentStatus when creating a payee or adding a contract, so no new request capability is introduced.
• Consumers cannot currently use this enum value; adding it simply documents existing API behaviour and fixes deserialization of GET responses.

Replaces the approach in #802 (which was flagged as a breaking change). This is a documentation/deserialization fix, not a functional change.