[ H4ck3r-netizen ] [ Crypto ] Fix PriceOracle missing staleness check and fallback mechanism

[H4ck3r-netizen]

Summary

Add staleness check, price validation, and fallback oracle to PriceOracle contract.

Changes

• Added round completeness validation: answeredInRound >= roundId
• Added price validation: rejects zero or negative prices
• Added staleness check: rejects prices older than MAX_STALENESS (3600 seconds)
• Added fallback oracle that is queried when primary returns stale/invalid data
• Emits StalePrice event when falling back to secondary oracle
• Reverts if both oracles return invalid data
• Constructor now accepts both primary and fallback feed addresses

Issue

Closes #915

Testing

• Valid price returned from primary oracle
• Stale primary price triggers fallback to secondary oracle
• Zero/negative prices rejected with clear error
• Incomplete rounds rejected
• Both oracles stale triggers revert
• MAX_STALENESS is configurable by owner

/bounty $200

[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.