feat(workitems): Implement UpdateWorkItem()

What does this MR do and why?

Implements UpdateWorkItem() on WorkItemsService, resolving #2221 (closed).

The method accepts a fullPath and iid (matching the conventions of GetWorkItem and ListWorkItems), performs an internal GID lookup via a new workItemGID() helper, and then issues a workItemUpdate GraphQL mutation.

UpdateWorkItemOptions is brought to feature parity with CreateWorkItemOptions, supporting all fields that can be set on creation:

Option	Widget
Title, StateEvent	top-level mutation input
Description	descriptionWidget
AssigneeIDs	assigneesWidget
MilestoneID	milestoneWidget
CRMContactIDs	crmContactsWidget (always REPLACE mode)
ParentID	hierarchyWidget
AddLabelIDs, RemoveLabelIDs	labelsWidget
StartDate, DueDate	startAndDueDateWidget
Weight	weightWidget
HealthStatus	healthStatusWidget
IterationID	iterationWidget
Color	colorWidget
Status	statusWidget

Several update-specific GQL input types are introduced where the GraphQL API requires a different shape than the create path (e.g. workItemWidgetLabelsUpdateInputGQL uses addLabelIds/removeLabelIds rather than a single set).

Also adds:

• WorkItemStatusID type and constants for the five system-defined statuses (ToDo, InProgress, Done, WontDo, Duplicate)
• WorkItemStateEvent type and CLOSE/REOPEN constants (previously untyped)
• gidGQL.IsZero() helper used to detect a missing work item in the ID-lookup response
• updateWorkItemTemplate for the mutation query, following the same template-chaining pattern as createWorkItemTemplate
• A TestUpdateWorkItem table-driven test covering: a simple title update, all options at once, not-found during ID lookup, null work item in mutation response, and a mutation-level error

All exported symbols carry the standard experimental API warning.