Add tests for GetGenHeader and GetParts methods in Msg

Introduce comprehensive tests for the GetGenHeader and GetParts methods. These tests cover single and multiple values for headers, body parts retrieval, and edge cases such as nil values, ensuring robustness and correctness.

msg_test.go

@@ -5,6 +5,7 @@
 package mail
 
 import (
+	"bytes"
 	"context"
 	"errors"
 	"fmt"
@@ -3068,6 +3069,144 @@ func TestMsg_GetBccString(t *testing.T) {
 	})
 }
 
+func TestMsg_GetGenHeader(t *testing.T) {
+	t.Run("GetGenHeader with single value", func(t *testing.T) {
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		for _, tt := range genHeaderTests {
+			t.Run(tt.name, func(t *testing.T) {
+				message.SetGenHeader(tt.header, "test")
+				values := message.GetGenHeader(tt.header)
+				if len(values) != 1 {
+					t.Errorf("GetGenHeader: expected 1 value, got: %d", len(values))
+				}
+				if values[0] != "test" {
+					t.Errorf("GetGenHeader: expected value not returned. Want: %s, got: %s",
+						"test", values[0])
+				}
+			})
+		}
+	})
+	t.Run("GetGenHeader with multiple values", func(t *testing.T) {
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		for _, tt := range genHeaderTests {
+			t.Run(tt.name, func(t *testing.T) {
+				message.SetGenHeader(tt.header, "test", "foobar")
+				values := message.GetGenHeader(tt.header)
+				if len(values) != 2 {
+					t.Errorf("GetGenHeader: expected 1 value, got: %d", len(values))
+				}
+				if values[0] != "test" {
+					t.Errorf("GetGenHeader: expected value not returned. Want: %s, got: %s",
+						"test", values[0])
+				}
+				if values[1] != "foobar" {
+					t.Errorf("GetGenHeader: expected value not returned. Want: %s, got: %s",
+						"foobar", values[1])
+				}
+			})
+		}
+	})
+	t.Run("GetGenHeader with nil", func(t *testing.T) {
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		for _, tt := range genHeaderTests {
+			t.Run(tt.name, func(t *testing.T) {
+				message.SetGenHeader(tt.header)
+				values := message.GetGenHeader(tt.header)
+				if len(values) != 0 {
+					t.Errorf("GetGenHeader: expected 1 value, got: %d", len(values))
+				}
+			})
+		}
+	})
+}
+
+func TestMsg_GetParts(t *testing.T) {
+	t.Run("GetParts with single part", func(t *testing.T) {
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		message.SetBodyString(TypeTextPlain, "this is a test body")
+		parts := message.GetParts()
+		if len(parts) != 1 {
+			t.Fatalf("GetParts: expected 1 part, got: %d", len(parts))
+		}
+		if parts[0] == nil {
+			t.Fatalf("GetParts: expected part, got nil")
+		}
+		if parts[0].contentType != TypeTextPlain {
+			t.Errorf("GetParts: expected contentType to be TypeTextPlain, got: %s", parts[0].contentType)
+		}
+		messageBuf := bytes.NewBuffer(nil)
+		_, err := parts[0].writeFunc(messageBuf)
+		if err != nil {
+			t.Errorf("GetParts: writeFunc failed: %s", err)
+		}
+		if !strings.EqualFold(messageBuf.String(), "this is a test body") {
+			t.Errorf("GetParts: expected message body to be %s, got: %s", "this is a test body",
+				messageBuf.String())
+		}
+	})
+	t.Run("GetParts with multiple parts", func(t *testing.T) {
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		message.SetBodyString(TypeTextPlain, "this is a test body")
+		message.AddAlternativeString(TypeTextHTML, "<p>This is HTML</p>")
+		parts := message.GetParts()
+		if len(parts) != 2 {
+			t.Fatalf("GetParts: expected 2 parts, got: %d", len(parts))
+		}
+		if parts[0] == nil || parts[1] == nil {
+			t.Fatalf("GetParts: expected parts, got nil")
+		}
+		if parts[0].contentType != TypeTextPlain {
+			t.Errorf("GetParts: expected contentType to be TypeTextPlain, got: %s", parts[0].contentType)
+		}
+		if parts[1].contentType != TypeTextHTML {
+			t.Errorf("GetParts: expected contentType to be TypeTextHTML, got: %s", parts[1].contentType)
+		}
+		messageBuf := bytes.NewBuffer(nil)
+		_, err := parts[0].writeFunc(messageBuf)
+		if err != nil {
+			t.Errorf("GetParts: writeFunc failed: %s", err)
+		}
+		if !strings.EqualFold(messageBuf.String(), "this is a test body") {
+			t.Errorf("GetParts: expected message body to be %s, got: %s", "this is a test body",
+				messageBuf.String())
+		}
+		messageBuf.Reset()
+		_, err = parts[1].writeFunc(messageBuf)
+		if err != nil {
+			t.Errorf("GetParts: writeFunc failed: %s", err)
+		}
+		if !strings.EqualFold(messageBuf.String(), "<p>This is HTML</p>") {
+			t.Errorf("GetParts: expected message body to be %s, got: %s", "<p>This is HTML</p>",
+				messageBuf.String())
+		}
+	})
+	t.Run("GetParts with no parts", func(t *testing.T) {
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		parts := message.GetParts()
+		if len(parts) != 0 {
+			t.Fatalf("GetParts: expected no parts, got: %d", len(parts))
+		}
+	})
+}
+
 // checkAddrHeader verifies the correctness of an AddrHeader in a Msg based on the provided criteria.
 // It checks whether the AddrHeader contains the correct address, name, and number of fields.
 func checkAddrHeader(t *testing.T, message *Msg, header AddrHeader, fn string, field, wantFields int,