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

[RaphaelFakhri]

Copies the collection arguments that CreateEmailOptions.Builder receives, so the builder no longer keeps a reference to the caller's list or map.

Problem

to(List<String>), tags(List<Tag>), attachments(List<Attachment>) and headers(Map<String, String>) store the argument as-is. The addTo, addTag, addAttachment and addHeader methods then mutate that same object:

• With an immutable collection, the follow-up call throws UnsupportedOperationException:

  CreateEmailOptions.builder()
      .to(List.of("a@example.com"))
      .addTo("b@example.com"); // throws

• With a mutable collection, addTo changes the caller's list, and later changes to the caller's list change the built options.

cc(List), bcc(List) and replyTo(List) already copy the argument, so the four methods behave differently from their siblings.

Solution

Store a copy in each of the four methods. The methods still replace the current value, as before, and null still clears it.

Tests

CreateEmailOptionsTest covers each method with an immutable argument followed by an add* call, and checks that to(List) does not write through to the caller's list. All five tests fail without the change and pass with it.

./gradlew test --tests "com.resend.services.emails.CreateEmailOptionsTest"

The full test task passes.

---

Summary by cubic

Fixes CreateEmailOptions.Builder storing the caller's to, tags, attachments, and headers collections by reference, so the builder now copies them instead.

Previously, addTo, addTag, addAttachment, and addHeader mutated the caller's collection: an immutable list such as List.of(...) caused UnsupportedOperationException, and a mutable list changed the caller's data. cc, bcc, and replyTo already copied their arguments, so the methods behaved inconsistently.

Adds CreateEmailOptionsTest, which covers immutable arguments followed by add* calls and confirms to(List) doesn't write through to the caller's list.

^{Written for commit dd70a94. Summary will update on new commits.}

[kewynakshlley]

Thanks for contributing @RaphaelFakhri