Update an existing contract type. Only provided fields will be updated.

Updatable Fields:

• name - Name of the contract type
• description - Description
• isActive - Active status
• primaryPipelineId - Primary pipeline reference
• secondaryPipelineId - Secondary pipeline reference
• referralTrackingEnabled - Referral tracking toggle

Milestone definitions and task templates are read-only through the Partner API.
Any attempt to write them returns 422 and no field in the request is saved. This
covers milestoneDefinitions and taskTemplates in camelCase or snake_case, the
_attributes variants, null, an object, and any array that differs from the
current value — including a reordered copy of it.

The one exception is a read-modify-write round trip: a milestoneDefinitions
array identical to what GET currently returns for this contract type (an empty
array on create, or on a contract type with no milestone definitions) is treated
as unchanged and ignored rather than rejected, in either key casing. Configure
these definitions in Contract Type settings.