test(builder): cover Builder lifecycle regressions

[krisoshea-eth]

Motivation

The Builder readiness, genesis polling, identity, and cancellation paths still lacked focused regression coverage after the initial Builder implementation. This PR adds that coverage without changing runtime behavior.

Changes

• Cover syncing, EL-offline, non-ok, unreachable, optimistic, and eventual-ready transitions.
• Prove cancellation during readiness and genesis polling stops further requests and timers.
• Keep node-version lookup failure non-fatal.
• Cover expected not-found genesis responses and unexpected genesis failures without noisy stack traces.
• Distinguish an empty successful Builder lookup, a non-active Builder status, and a failed beacon node response.
• Preserve useful typed API error detail on status lookup failure.
• Cover cancellation while waiting for an unknown Builder to appear.
• Extend the Builder API stub with getGenesis and getNodeVersionV2.

This is a test-only change. It does not alter polling intervals, logs, API contracts, public exports, or Builder runtime behavior.

Historical validation base: f22c5ce63e, including merged #9832 and #9964.

Reviewed head on 2026-09-07: 9898d54ca9 (source unchanged in this pass).

Project issue: krisoshea-eth/lodestar#45.

Testing

Previously recorded validation with Node 24.13.0 and pnpm 11.0.0:

• 29 focused readiness, genesis, and identity tests
• all 40 Builder unit tests
• 8 existing Builder CLI and fee-recipient parser tests
• Builder package type-check
• Builder package lint
• git diff --check

The Builder coverage comprises 7 readiness tests, 3 genesis tests, 19 identity tests, 5 BuilderStatusTracker tests, and 6 signer tests. Existing CLI tests continue to cover zero fee-recipient and unscheduled Gloas validation.

Earlier CI results belong to their recorded heads. See the Checks tab for runs on the current head; absent runs or runs awaiting approval must not be treated as passing. This review did not refresh the branch from unstable.

AI assistance disclosure

• External Contributors: I have read the contributor guidelines and disclosed my usage of AI below.

AI assistance was used for codebase research, test drafting, validation, and review. I reviewed and revised the submitted tests and PR text, made manual edits and technical decisions, and take responsibility for the final result.

[markolazic01]

maybe change the title here, Gate A is an unknown term for external viewers.

[markolazic01]

LGTM

[nflaig]

LGTM, thanks @krisoshea-eth and sorry for the delay

[wemeetagain]

🎉 This PR is included in v1.48.0 🎉