[OpenAI Codex] [ Crypto ] Fix PriceOracle validation and fallback

[justusaugust]

/claim #915

Summary

• Validate Chainlink round completeness, positive prices, and nonzero timestamps before returning a price.
• Add stale-price fallback support with owner-configurable staleness and fallback feed updates.
• Emit StalePrice when the primary feed is stale and add a Solidity runtime test for valid, stale fallback, both stale, negative, incomplete, and owner-only configuration paths.

Verification

• npm install --no-package-lock
• npm test

Demo Video

• Short demo video for this PR

[github-actions]

Unfortunately the changes in this PR didn't fully resolve the issue. Please rework your solution and submit a new pull request.

Make sure to review the acceptance criteria in the linked issue and verify all conditions are met before resubmitting. See CONTRIBUTING.md for guidelines.