Add App API segment & subscription methods (CDP-6266)

[mike-engel]

Batch 2 of the App API backfill. Adds 10 read/query methods to APIClient:

┌────────────────────────────┬───────────────────────────────────────────┐
│           Method           │                 Endpoint                  │
├────────────────────────────┼───────────────────────────────────────────┤
│ listSegments               │ GET /segments                             │
├────────────────────────────┼───────────────────────────────────────────┤
│ createSegment              │ POST /segments                            │
├────────────────────────────┼───────────────────────────────────────────┤
│ getSegment                 │ GET /segments/{id}                        │
├────────────────────────────┼───────────────────────────────────────────┤
│ deleteSegment              │ DELETE /segments/{id}                     │
├────────────────────────────┼───────────────────────────────────────────┤
│ getSegmentCustomerCount    │ GET /segments/{id}/customer_count         │
├────────────────────────────┼───────────────────────────────────────────┤
│ getSegmentMembership       │ GET /segments/{id}/membership (paginated) │
├────────────────────────────┼───────────────────────────────────────────┤
│ getSegmentUsedBy           │ GET /segments/{id}/used_by                │
├────────────────────────────┼───────────────────────────────────────────┤
│ listSubscriptionTopics     │ GET /subscription_topics                  │
├────────────────────────────┼───────────────────────────────────────────┤
│ listSubscriptionChannels   │ GET /subscription_channels                │
├────────────────────────────┼───────────────────────────────────────────┤
│ getSubscriptionCenterToken │ GET /subscription_center/{id}/token       │
└────────────────────────────┴───────────────────────────────────────────┘

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Note

Medium Risk
Adds destructive segment APIs (deleteSegment) and subscription-center tokens; behavior is thin HTTP wrappers with tests, but callers can mutate workspace segments with a valid app key.

Overview
Adds 10 new APIClient methods (CDP-6266 batch 2) so the Node SDK can call segment and subscription App API endpoints that were not previously wrapped.

Segments: listSegments, createSegment (with new SegmentInput), getSegment, deleteSegment, getSegmentCustomerCount, paginated getSegmentMembership, and getSegmentUsedBy. Mutating calls use the same validation style as other client methods (MissingParamError, URL encoding).

Subscriptions: listSubscriptionTopics, listSubscriptionChannels, and getSubscriptionCenterToken for hosted preference links.

docs/app.md documents each method; unit tests assert URLs, payloads, and validation; live integration tests cover list/read paths plus a create→read→delete segment round-trip.

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