fix(check): allow missing caption overrides

[miguel-heygen]

What

Treat a missing root caption-overrides.json as an optional runtime resource in both check and validate.

Why

The caption runtime intentionally fetches overrides only when caption groups exist and safely handles a missing file, but browser diagnostics converted the expected 404/aborted request into http_error and request_failed. Users had to create an empty JSON shim before checks could pass.

How

• ignore only the exact root /caption-overrides.json request when it is absent
• keep other JSON failures and non-404 caption override responses reportable
• share the same predicates across check and validate

Test plan

• vitest run src/commands/validate.test.ts src/utils/checkBrowser.test.ts (27 passed)
• tsc --noEmit
• patched CLI against minimal caption composition with no override file: ok:true, zero runtime findings
• repository pre-commit lint, format, fallow, tracked-artifact, and typecheck hooks passed