[runtime] tighten verify warmup polling and centralize cached-only warmup

[GsCommand]

Motivation

• Remove hidden timing assumptions from the full-chain schema verification test so it models cached-only validator warmup reliably.
• Consolidate cached-only validator warmup entrypoint inside the runtime to reduce duplicated queue/start logic.
• Preserve the /verify public contract while making failures clearer and deterministic.

Description

• Update the full-chain test runtime/tests/runtime-signing.test.mjs to replace fragile single-retry logic with a bounded 3-attempt polling loop that treats 202 + validator_not_warmed_yet as an expected warmup state and respects retry_after_ms with a 1000ms fallback.
• Increase request-level AbortController timeout for /verify calls in the test to 10000ms to avoid spurious aborts during warmup.
• Keep strict final assertions (require verifyJson.checks.signature_valid === true, hash_matches === true, and schema_valid === true) and fail clearly if warmup never completes or an unexpected response is returned.
• Replace manual warmQueue.add(verb) / startWarmWorker() calls in the /verify cached-only path with the canonical warmValidatorForVerb(verb) helper while preserving the 202 status, response body shape, schema_errors payload, and retry_after_ms behavior.

Testing

• Ran node --test runtime/tests/runtime-signing.test.mjs and the test suite completed successfully with the full-chain test passing.
• No other repo files were modified and existing verification/signing semantics and public /verify contract remain unchanged.

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Codex Task