Implement oracle slashing for bad data providers

[Kevin737866]

Description

Penalize oracle nodes that submit provably incorrect data by slashing their stake, maintaining oracle network integrity.

Acceptance Criteria

• Slashing for data more than X% from consensus (configurable)
• Slashed funds redistributed to honest oracles
• Challenge period before slashing executed
• Multiple severity levels for different deviations
• Slashing events with evidence

Files to Modify

• src/contracts/decentralized_oracle.rs -- add slashing
• src/contracts/staking.rs -- integrate slashing for oracle stakers
• tests/ -- slashing tests