fix: inbound attachment types

[gabrielmfern]

Signed-off-by: gabriel miranda gabrielmfern@outlook.com

---

Summary by cubic

Align inbound and signed-URL attachment types with the API to remove response type mismatches. Includes nullable inbound fields per DEV-934 and adds a clear model for signed URL responses.

• Bug Fixes
  • Added AttachmentWithSignedUrl for list/retrieve attachment endpoints, including download_url, expires_at, required size, and content_disposition as "inline" or "attachment".
  • Updated EmailAttachment for inbound emails to allow filename, content_id, and content_disposition to be null; size is optional/nullable.
  • Switched list responses in resend.emails._attachments and resend.emails._receiving to return List[AttachmentWithSignedUrl].
  • Exported AttachmentWithSignedUrl in resend/__init__.py.
  • Extended tests to cover download_url/expires_at and nullable inbound attachment fields.

^{Written for commit 6a6494c. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 9 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

<sub>Requires human review: This PR restructures attachment type definitions (renaming models, changing field types to Literal and NotRequired, and moving fields), which is a refactor that could break existing SDK consumers relying on the previous type shapes.

Re-trigger cubic</sub>