NYL-22: Add Contact metadata and webhook parity

[quzhi1]

Summary

• add Contact metadata request/response models and metadata_pair contact filtering
• document metadata replace, preserve, and clear behavior plus provider webhook support boundaries
• verify the existing contact.updated and contact.deleted wire values and guard against unsupported contact.created

Test plan

• /Users/zhi.q/nylas-python/.venv/bin/python -m pytest (485 passed, 3 deselected)

[AaronDDM]

Good morning! Looks good — models, docstrings, changelog and tests all covered, and the enum already had the contact triggers so the docstring-only change there is right.

One suggestion below. Approving.

[AaronDDM]

Good morning! Re-reviewed after 6406a73.

The _build_query_params test is exactly what was missing — that's the piece that could have silently regressed. UpdateContactRequest is an alias of CreateContactRequest, so metadata covers both paths. CHANGELOG has an Unreleased entry and the docstrings cover the update semantics.

One non-blocking cross-SDK note below. Approving.

[AaronDDM]

Non-blocking, just noting it: Python emits the colon raw (metadata_pair=key1:sync_eligible) while Node and Ruby percent-encode it (key1%3Async_eligible). Both are valid per RFC 3986 and the API decodes either, so nothing to change here — it's pre-existing serializer behavior in each SDK, not something this PR introduced. Flagging it in case it ever comes up in a support ticket.