refactor!: remove AttachmentBuilder and support new file body encodables

[didinele]

• Depends on feat(builders): multipart form data output support #11248

Closes #11273

[vercel]

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2 Skipped Deployments

Project	Deployment	Preview	Comments	Updated (UTC)
discord-js	Skipped			Nov 29, 2025 2:13pm
discord-js-guide	Skipped			Nov 29, 2025 2:13pm

[didinele]

Ready for review, but blocked until the other PR is merged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.97%. Comparing base (68bb8af) to head (597310a).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #11278   +/-   ##
=======================================
  Coverage   44.97%   44.97%           
=======================================
  Files         317      317           
  Lines       18222    18222           
  Branches     1817     1817           
=======================================
  Hits         8196     8196           
  Misses      10013    10013           
  Partials       13       13           

Flag	Coverage Δ
builders	81.15% <ø> (ø)

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Resolved.