Characterize new-format webhook handling

[camdroid]

Extends the legacy characterization suite (asmbly-makerspace#100) to the new (post 2026-05-09)
Neon webhook format.

Green — current behavior:

• new editAccount / updateMembership: account id resolved and passed straight to
  OpenPath as a string (id-type divergence from legacy noted inline).
• new createMembership RENEW / FAILED: correctly skip join handling, like legacy.

xfail(strict) — target behavior for the unification refactor:

• new createMembership JOIN/REJOIN (SUCCEEDED) should enter handle_joins.
• a fresh new-format join should add the member to Mailjet.
  Both fail today (flat enrollType/status aren't recognized, so new joins are
  silently dropped) and will XPASS — failing CI — once the refactor routes new
  joins through handle_joins, signaling the markers can be removed.

Note: new createMembership uses an ASSUMED shape (no real sample in the logs),
inferred from the new updateMembership shape.