#300 fix: allow null optional inbox params

Summary

• Mark optional nullable Inbox reply and forward fields as accepting None.
• Mark nullable draft attachment and send-header fields as accepting None.
• Add a regression test covering all affected fields.

Verification

• Regression fixture produced 10 mypy errors before the annotation fix.
• tox: format, lint, mypy, and pytest passed (852 tests; 10 existing deprecation warnings).
• Re-ran lint, mypy, and pytest after removing unrelated formatter output; all passed.

---

Summary by cubic

Updates the inbox SDK type hints so optional reply, forward, and draft parameters accept None instead of only being omittable. Affected fields include cc, bcc, headers, and attachments on reply and forward params, plus attachments on draft create params and headers on draft send params. Adds a regression test covering these fields.