fix: Handle null assignee in Copilot Seat Billing API response (#3619)

Author: sanjayy-gowdaa

github/copilot.go

@@ -203,6 +203,11 @@ func (cp *CopilotSeatDetails) UnmarshalJSON(data []byte) error {
 	cp.PlanType = seatDetail.PlanType
 
 	switch v := seatDetail.Assignee.(type) {
+	case nil:
+		// Assignee can be null according to GitHub API specification.
+		// See: https://docs.github.com/en/rest/copilot/copilot-user-management?apiVersion=2022-11-28#list-all-copilot-seat-assignments-for-an-organization
+		// Note: Copilot API is in public preview and subject to change.
+		cp.Assignee = nil
 	case map[string]any:
 		jsonData, err := json.Marshal(seatDetail.Assignee)
 		if err != nil {

github/copilot_test.go

@@ -58,6 +58,16 @@ func TestCopilotSeatDetails_UnmarshalJSON(t *testing.T) {
 			want:    &CopilotSeatDetails{},
 			wantErr: true,
 		},
+		{
+			name: "Null Assignee",
+			data: `{
+					"assignee": null
+				}`,
+			want: &CopilotSeatDetails{
+				Assignee: nil,
+			},
+			wantErr: false,
+		},
 		{
 			name: "Invalid Assignee Field Type",
 			data: `{