DX-2104: make schema tests hermetic (no live spec fetch)

[lloyd-cio]

Problem

DX-2104: the cio schema tests run the real command, which loads the route registry by downloading the OpenAPI specs from a live endpoint (us.fly.customer.io). The public-export validation runs go test ./... over the exported tree, so a transient network reset on that fetch fails the whole export:

TestSchema_ListResources: failed to load API specs:
  read https://us.fly.customer.io/v1/openapi.json: connection reset by peer
FAIL go test ./...

(Seen blocking the DX-2103 export verification, run 27435049667.)

Fix (test-only, no production changes)

Add setupSchemaTest: a local httptest spec server serving Journeys + CDP fixtures (with a campaigns resource, so the assertions resolve) plus an isolated temp HOME cache, wired in through CIO_API_URL. Point the five TestSchema_* tests at it.

schema.go is the only production consumer of routes.LoadRegistry, and the override seams already existed (CIO_API_URL → client BaseURL → LoadRegistryOptions.BaseURL; temp HOME isolates the spec cache), so no production code changed.

Verification

• go test ./... passes normally.
• Offline proof: go test ./... passes with all external HTTP(S) routed to a dead proxy (HTTPS_PROXY=http://127.0.0.1:1, localhost bypassed) and GOPROXY=off — confirming no test reaches the network. A control curl to the real host through that proxy fails as expected.

This makes the export validator's go test hermetic, so exports no longer flake on network.