NYL-22: Add Contact metadata and webhook parity

[quzhi1]

Summary

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

Test plan

• npm test -- --coverage=false (488 tests passed)
• npm run build
• Prettier on changed files

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.75%. Comparing base (2bdf4ae) to head (881a3c4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #759   +/-   ##
=======================================
  Coverage   98.75%   98.75%           
=======================================
  Files          43       43           
  Lines        1041     1041           
  Branches       90       90           
=======================================
  Hits         1028     1028           
  Misses         12       12           
  Partials        1        1           

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[AaronDDM]

Good morning! Nice clean change — types, docs, changelog and tests are all here, and the contact.created guard test is a good touch.

Two notes below, neither blocking. Approving.

[AaronDDM]

Good morning! Re-reviewed after 881a3c4.

Both items from yesterday are addressed:

• tests/apiClient.spec.ts now asserts the real outgoing URL (metadata_pair=key1%3Async_eligible), which was the gap.
• The multi-entry join in apiClient.ts is pre-existing shared-serializer behavior and out of scope here — agreed on leaving it.

CHANGELOG has an Unreleased entry, doc comments cover the replace/preserve/clear semantics, and UpdateContactRequest picks up metadata via the alias to CreateContactRequest. No new concerns.

Approving.