fix(emails): copy list and map arguments in CreateEmailOptions builder

src/main/java/com/resend/services/emails/model/CreateEmailOptions.java

@@ -246,7 +246,7 @@ public Builder to(String... recipients) {
          * @return This builder instance for method chaining.
          */
         public Builder to(List<String> recipients) {
-            this.to = recipients;
+            this.to = recipients == null ? null : new ArrayList<>(recipients);
             return this;
         }
 
@@ -436,7 +436,7 @@ public Builder html(String html) {
          * @return This builder instance for method chaining.
          */
         public Builder headers(Map<String, String> headers) {
-            this.headers = headers;
+            this.headers = headers == null ? null : new HashMap<>(headers);
             return this;
         }
 
@@ -478,7 +478,7 @@ public Builder attachments(Attachment... attachments) {
          * @return This builder instance for method chaining.
          */
         public Builder attachments(List<Attachment> attachments) {
-            this.attachments = attachments;
+            this.attachments = attachments == null ? null : new ArrayList<>(attachments);
             return this;
         }
 
@@ -519,7 +519,7 @@ public Builder tags(Tag... tags) {
          * @return This builder instance for method chaining.
          */
         public Builder tags(List<Tag> tags) {
-            this.tags = tags;
+            this.tags = tags == null ? null : new ArrayList<>(tags);
             return this;
         }
 

src/test/java/com/resend/services/emails/CreateEmailOptionsTest.java

@@ -0,0 +1,80 @@
+package com.resend.services.emails;
+
+import com.resend.services.emails.model.Attachment;
+import com.resend.services.emails.model.CreateEmailOptions;
+import com.resend.services.emails.model.Tag;
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class CreateEmailOptionsTest {
+
+    private static CreateEmailOptions.Builder base() {
+        return CreateEmailOptions.builder().from("sender@example.com").subject("Subject");
+    }
+
+    @Test
+    public void toListAcceptsImmutableListAndAllowsAddTo() {
+        CreateEmailOptions options = base()
+                .to(Collections.singletonList("a@example.com"))
+                .addTo("b@example.com")
+                .build();
+
+        assertEquals(Arrays.asList("a@example.com", "b@example.com"), options.getTo());
+    }
+
+    @Test
+    public void toListDoesNotWriteThroughToCallerList() {
+        List<String> recipients = new ArrayList<>(Collections.singletonList("a@example.com"));
+
+        CreateEmailOptions options = base().to(recipients).addTo("b@example.com").build();
+        recipients.add("c@example.com");
+
+        assertEquals(Arrays.asList("a@example.com", "c@example.com"), recipients);
+        assertEquals(Arrays.asList("a@example.com", "b@example.com"), options.getTo());
+    }
+
+    @Test
+    public void tagsListAcceptsImmutableListAndAllowsAddTag() {
+        Tag first = Tag.builder().name("a").value("1").build();
+        Tag second = Tag.builder().name("b").value("2").build();
+
+        CreateEmailOptions options = base()
+                .tags(Collections.singletonList(first))
+                .addTag(second)
+                .build();
+
+        assertEquals(Arrays.asList(first, second), options.getTags());
+    }
+
+    @Test
+    public void attachmentsListAcceptsImmutableListAndAllowsAddAttachment() {
+        Attachment first = Attachment.builder().fileName("a.txt").content("YQ==").build();
+        Attachment second = Attachment.builder().fileName("b.txt").content("Yg==").build();
+
+        CreateEmailOptions options = base()
+                .attachments(Collections.singletonList(first))
+                .addAttachment(second)
+                .build();
+
+        assertEquals(Arrays.asList(first, second), options.getAttachments());
+    }
+
+    @Test
+    public void headersMapAcceptsImmutableMapAndAllowsAddHeader() {
+        CreateEmailOptions options = base()
+                .headers(Collections.singletonMap("X-One", "1"))
+                .addHeader("X-Two", "2")
+                .build();
+
+        Map<String, String> headers = options.getHeaders();
+        assertEquals("1", headers.get("X-One"));
+        assertEquals("2", headers.get("X-Two"));
+    }
+}