Agent registration: route-level integration tests

[arturasmckwcz]

Follow-up from #601.

The write helpers have 14 unit tests, but register.routes and membership.routes were only smoke-tested manually. Add automated integration tests covering:

• POST /agent/register: token auth (400/401/403), create (201), join (active/pending), 409 + agentId.
• GET /agent/register/search: token gate + domain-scoped results.
• GET/PATCH/DELETE /agent/membership: COORDINATOR/ADMIN guard, approve, reject, 404.

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