Dedupe builder/validator test `apiStub` helpers into `packages/test-utils`

[lodekeeper]

Follow-up from #9781.

packages/builder/test/unit/utils/apiStub.ts (added in #9781) largely duplicates packages/validator/test/utils/apiStub.ts. Per discussion on #9781, the shared parts should be deduped into packages/test-utils as a follow-up so the two don't drift.

Current state

• packages/builder/test/unit/utils/apiStub.ts (on the feat(builder): add readiness gating and builder identity resolution #9781 branch)
• packages/validator/test/utils/apiStub.ts (unstable)

Comparing the two, it's a bit more than a 1:1 dup — a few things need reconciling:

• mockApiResponse — identical in both; trivially shareable.
• getApiClientStub — function lists differ. Builder stubs only beacon.getStateBuilders + node.getSyncingStatus; validator stubs a much larger beacon/node/validator set plus httpClient. Note the builder adds beacon.getStateBuilders, which the validator stub doesn't have. A shared helper would need to expose a superset or be parameterized so each consumer only stubs the endpoints it needs.
• mockApiErrorResponse — already aligned on unstable: the validator version is async + await res.errorBody() there. (An earlier revision of this issue flagged it as diverged, but that compared against the feat(builder): add readiness gating and builder identity resolution #9781 branch snapshot, which predates that change — thanks @markolazic01.) Once feat(builder): add readiness gating and builder identity resolution #9781 is on top of unstable, both are identical, so no reconciliation needed here.
• httpClientStub + httpClient field — present in validator, omitted in builder.
• ApiClientStub type — exported in builder, local in validator.

Suggested approach

Move the shared pieces (the ApiClientStub type, mockApiResponse, mockApiErrorResponse, and a base getApiClientStub / httpClientStub) into packages/test-utils, and have both the builder and validator suites import from there, stubbing package-specific endpoints on top as needed.

Not urgent — filing so it isn't lost after #9781 merges.

Refs: #9781, #9781 (comment)

[markolazic01]

this doesn't affect things much but you patched it on the validator already, mockApiErrorResponse is already async on both ends

[lodekeeper]

You're right — thanks. I'd compared the builder copy against the validator file on the #9781 branch, which is behind unstable on that helper (validator's mockApiErrorResponse is already async + await res.errorBody() on unstable). So once #9781 rebases on unstable they match — updated the issue to drop that as a reconciliation point.