Extract shared HTTP logic into doHTTP

[hownowstephen]

Summary

• CustomerIO.request() and APIClient.doRequest() duplicated the same marshal → create request → set headers → execute → read body pipeline
• Extracts doHTTP() in http.go that accepts an auth callback; both clients delegate to it
• Auth (Basic vs Bearer) and error handling remain caller-specific
• Also fixes minor bug: CustomerIO.request() was not setting User-Agent header on DELETE requests (nil body path)
• Net -19 lines

Test plan

• go test -race ./... passes
• No public API changes — internal refactor only

---

Note

Low Risk
Internal refactor with no public API changes; behavior is intended to stay the same aside from the User-Agent fix on nil-body Track requests.

Overview
Consolidates duplicated HTTP request logic from CustomerIO.request() and APIClient.doRequest() into a new shared doHTTP helper in http.go. Both clients now delegate marshaling, request construction, User-Agent, auth via a caller preflight hook (Basic vs Bearer), execution, and body reads to that path while keeping their own status/error handling.

Also fixes CustomerIO requests with a nil body (e.g. DELETE) so User-Agent is set consistently, which the old nil-body branch skipped.

^{Reviewed by Cursor Bugbot for commit ce5caeb. Bugbot is set up for automated code reviews on this repo. Configure here.}