[runtime] prefer builtin schemas during validator warmup

[GsCommand]

Motivation

• Cached-only /verify warmup could still hit the network before falling back to builtins, causing CI flakiness when builtin schemas are available.
• Implement a builtin-first warmup path to make validator precompilation deterministic while preserving the public /verify contract.

Description

• Add options support to fetchJsonWithTimeout(url, timeoutMs, options = {}) and implement preferBuiltin: true which normalizes the URL then uses getBuiltinSchema() first and caches/returns it if present.
• Thread the option through makeAjv(options = {}) and getValidatorForVerb(verb, options = {}), using fetchJsonWithTimeout(..., options) for shared refs and main schema loads.
• Modify the warm worker to call getValidatorForVerb(verb, { preferBuiltin: true }) so cached-only warmup compiles validators using builtin schemas before any network fetch.
• Preserve existing semantics for validatorCache and inflightValidator, and leave default validation paths unchanged when preferBuiltin is omitted.

Testing

• Ran node --test runtime/tests/runtime-signing.test.mjs; all tests passed (subtests covering the full verify warmup chain succeeded).
• Ran npm test (unit tests + smoke); all tests and smoke checks passed.

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Codex Task