test(events): add comprehensive marshal/unmarshal tests for event types (#98)

Add detailed tests for DeliveryEvent, OpenEvent, ClickEvent, BounceEvent,
and SpamComplaintEvent to verify correct JSON marshaling and unmarshaling.

These tests improve coverage and ensure event structs properly serialize and
deserialize, preventing regressions in event data handling.

Author: mrz1836

email_test.go

@@ -45,6 +45,7 @@ func (s *PostmarkTestSuite) TestSendEmail() {
 		responseJSON string
 		wantErr      bool
 		expectedID   string
+		statusCode   int
 	}{
 		{
 			name: "successful email send",
@@ -57,6 +58,7 @@ func (s *PostmarkTestSuite) TestSendEmail() {
 			}`,
 			wantErr:    false,
 			expectedID: "0a129aee-e1cd-480d-b08d-4f48548ff48d",
+			statusCode: http.StatusOK,
 		},
 		{
 			name: "email send failure with error code",
@@ -67,13 +69,26 @@ func (s *PostmarkTestSuite) TestSendEmail() {
 				"ErrorCode": 401,
 				"Message": "Sender signature not confirmed"
 			}`,
-			wantErr: true,
+			wantErr:    true,
+			statusCode: http.StatusOK,
+		},
+		{
+			name: "email send HTTP error",
+			responseJSON: `{
+				"ErrorCode": 500,
+				"Message": "Internal Server Error"
+			}`,
+			wantErr:    true,
+			statusCode: http.StatusInternalServerError,
 		},
 	}
 
 	for _, tt := range tests {
 		s.Run(tt.name, func() {
 			s.mux.Post("/email", func(w http.ResponseWriter, _ *http.Request) {
+				if tt.statusCode != http.StatusOK {
+					w.WriteHeader(tt.statusCode)
+				}
 				_, _ = w.Write([]byte(tt.responseJSON))
 			})