Add App API customer, object, activity, and segment read methods

[oke11o]

Summary

APIClient currently only exposes the write/send side of the App API (SendEmail, SendPush, SendSMS, SendInApp, SendInboxMessage, TriggerBroadcast). This adds the read-side methods that the Node SDK's APIClient already covers:

• Customers: SearchCustomers, GetCustomersByEmail, GetAttributes, GetCustomerActivities, GetCustomerMessages, GetCustomerRelationships, GetCustomerSegments, GetCustomerSubscriptionPreferences, GetCustomersAttributes (bulk), GetSubscriptionCenterToken
• Objects: GetObjectAttributes, GetObjectRelationships, FindObjects, ListObjectTypes
• Activities: ListActivities
• Segments: ListSegments, CreateSegment, GetSegment, DeleteSegment, GetSegmentCustomerCount, GetSegmentMembership, GetSegmentUsedBy
• Subscriptions: ListSubscriptionTopics, ListSubscriptionChannels

SearchCustomers and FindObjects take a composable filter (AudienceFilter/ObjectFilter, built via FilterBySegment/FilterByAttribute/FilterAnd/FilterOr/FilterNot and their ObjectFilter* counterparts) mirroring the Node SDK's AudienceFilter/ObjectFilter union types, flattened to a single struct since Go has no union types.

Notes on shapes

• Request shapes are cross-checked against customerio-node's lib/api.ts/lib/types.ts.
• Response shapes are cross-checked against the published App API OpenAPI spec. A few fields are documented inconsistently as string vs. integer across endpoints (or between schema and example) — e.g. object_type_id, a subscription topic's id. Those are decoded via json.Number rather than a fixed type so either representation round-trips.
• GetCustomerMessages/GetCustomerActivities accept an IDType option (id/email/cio_id), matching the Node SDK — this lets a caller look a customer up by email directly instead of a separate search call first.
• Endpoints without a documented example response (only a schema) are modeled from the schema field names/types as declared, flagged in code comments where relevant.

Test plan

• go build ./...
• go vet ./...
• go test ./...
• golangci-lint run ./... (v2.11.3, matching this repo's CI)

---

Note

Low Risk
Additive SDK surface and shared HTTP helpers; segment delete is the only mutating new call, with validation and tests. No changes to existing send/auth paths.

Overview
Expands APIClient beyond send/trigger APIs with read-side Customer.io App API coverage aligned to the Node SDK: customer search and profiles, objects, workspace activities, segments (including create/delete), and subscription topic/channel listing.

Shared plumbing adds doJSON on APIClient (marshal/unmarshal plus configurable success statuses), a queryBuilder in url.go that omits zero-value query params, PaginationOptions, and composable AudienceFilter / ObjectFilter helpers for SearchCustomers and FindObjects. Several response fields use json.Number where the API inconsistently returns string vs integer IDs.

HTTP tests use a new apiServer helper that records request method, path, and body for assertions.

^{Reviewed by Cursor Bugbot for commit 2784f7d. Bugbot is set up for automated code reviews on this repo. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{Reviewed by Cursor Bugbot for commit 2ad89c4. Configure here.}