[runtime] tighten full-chain schema warmup retry test

[GsCommand]

Motivation

• Make the full-chain schema verification test more reliable in CI by forcing the background validator warmup to fail fast to builtin schemas so the existing test retry logic can succeed without changing runtime behavior.

Description

• Updated runtime/tests/runtime-signing.test.mjs test env to keep VERIFY_SCHEMA_CACHED_ONLY: "1", VERIFY_MAX_MS: "12000", set SCHEMA_FETCH_TIMEOUT_MS: "50", and keep SCHEMA_VALIDATE_BUDGET_MS: "3000" so schema fetch falls back quickly.
• Reworked the verify retry to preserve the existing logging and polling flow but retry up to two times while honoring retry_after_ms from the 202 response with a 1200ms fallback.
• Preserved the final assertion that once /verify?schema=1 returns 200, checks.schema_valid, checks.signature_valid, and checks.hash_matches must be true.

Testing

• Ran node --test runtime/tests/runtime-signing.test.mjs and all tests passed locally.

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Codex Task