[TDT-2810] V6.18.0 release

[benjaminwhtan]

https://nylas.atlassian.net/browse/TDT-2810

Changelog (v6.18.0)

• Added optional tracking_options.domain_name support for custom link and open tracking hostnames in message sends, scheduled sends, drafts, and Transactional Send (Add custom tracking hostname support to Python SDK #484, EN-143)
• Added Contact metadata request/response models and metadata_pair filtering, with contact webhook compatibility documentation (NYL-22: Add Contact metadata and webhook parity #485, NYL-22)
• Added messages.send_raw_mime() to send a raw RFC 822 MIME message via POST /v3/grants/{grant_id}/messages/send?type=mime, with the SendRawMimeRequest model ([CUST-5735] feat(messages): add send_raw_mime for raw MIME message send #488, CUST-5735)

Release process notes

This release follows the release runbook with these changes:

• No tag on this branch. The repo only allows squash merges, so a tag on the release branch never reaches main. v6.16.0 and v6.17.0 both point to commits that aren't on main. v6.18.0 will be tagged on this PR's squash commit after merge.
• Publish after merge. PyPI and TestPyPI can't re-upload a version, so artifacts are built from main only after this PR is approved and merged.
• python -m build replaces python setup.py sdist, so the release ships a wheel as well as the source package.
• Staging check: the TestPyPI build is installed in a clean venv and tested against the staging API before the prod upload.

Testing

• Local python -m build: nylas-6.18.0.tar.gz and nylas-6.18.0-py3-none-any.whl built, and twine check passes on both
• The wheel only contains nylas/ and includes send_raw_mime. The source package layout matches the published 6.17.0.
• CI passes
• TestPyPI install + raw MIME send on a staging Microsoft grant (after merge)

License

I confirm that this contribution is made under the terms of the MIT license and that I have the authority necessary to make this contribution on behalf of its copyright owner.

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