[Kayvan-Zahiri]

Fixes #980

The shared handwritten ISO8601 datetime serializer currently discards UTC offsets while appending Z. For example, a scheduled-message send_at of 2030-01-02 01:30+05:30 becomes 2030-01-02T01:30:00Z, shifting the requested instant by 5.5 hours.

Convert aware datetimes to UTC before formatting. Keep the existing naive/date/string/unset behavior. Document the conversion in the serializer and add six tests through the public Client.messages.create API using a mocked HTTP transport, including positive/negative offsets across date boundaries and four compatibility controls. No generated REST files are changed.

Validation: two regressions fail before the fix and four controls pass. Full required make test passes on Python3.12:647 tests, source/test Flake8, Autoflake, and Black (797 files). No live Twilio requests/messages or full Python matrix run. AI-assisted contribution; maintainer review pending.

Checklist

• Contributions are made under the project's MIT license.
• Material functionality and test change.
• Read and followed the Contribution Guidelines.
• PR title follows the required format.
• Based on the current main checkout.
• Added tests proving the fix.
• Added appropriate inline documentation; no new public API requires separate Markdown documentation.