add missing WithMergeFragmentsEventID option

The merge fragments call configuration struct had an EventID field and
logic that added it to the SSE call, but no option to configure it.
Added that missing option.

sdk/go/fragments.go

@@ -20,6 +20,17 @@ type mergeFragmentOptions struct {
 // MergeFragmentOption configures the [sse.MergeFragments] event initialization.
 type MergeFragmentOption func(*mergeFragmentOptions)
 
+// WithMergeFragmentsEventID configures an optional event ID for the fragments merge event.
+// The client message field [lastEventId] will be set to this value.
+// If the next event does not have an event ID, the last used event ID will remain.
+//
+// [lastEventId]: https://developer.mozilla.org/en-US/docs/Web/API/MessageEvent/lastEventId
+func WithMergeFragmentsEventID(id string) MergeFragmentOption {
+	return func(o *mergeFragmentOptions) {
+		o.EventID = id
+	}
+}
+
 // WithSelectorf is a convenience wrapper for [WithSelector] option that formats the selector string
 // using the provided format and arguments similar to [fmt.Sprintf].
 func WithSelectorf(selectorFormat string, args ...any) MergeFragmentOption {
@@ -57,7 +68,7 @@ func WithUseViewTransitions(useViewTransition bool) MergeFragmentOption {
 // MergeFragments sends an HTML fragment to the client to update the DOM tree with.
 func (sse *ServerSentEventGenerator) MergeFragments(fragment string, opts ...MergeFragmentOption) error {
 	options := &mergeFragmentOptions{
-		EventID:       "", // TODO: Implement EventID option? currently field does nothing.
+		EventID:       "",
 		RetryDuration: DefaultSseRetryDuration,
 		Selector:      "",
 		MergeMode:     FragmentMergeModeMorph,