Add Contract guarantees update method

[evanvolgas]

Summary

The SDK is missing the ability to update contract guarantees after publishing.

Core API endpoint

• PATCH /contracts/{id}/guarantees - Update guarantees on an existing contract

Method to add

class ContractsResource:
    def update_guarantees(
        self, 
        contract_id: UUID, 
        guarantees: Guarantees
    ) -> Contract:
        """Update the guarantees (SLAs) on a published contract."""

Use case

Updating freshness SLAs or other guarantees without republishing the schema.