Implement oracle update batching for gas efficiency

[Kevin737866]

Description

Batch multiple oracle price updates into a single transaction to reduce gas costs for oracle operators and improve update frequency.

Acceptance Criteria

• Update up to 50 asset prices in one transaction
• Gas cost per asset reduced by 40%+ in batch mode
• Individual validation per asset in batch
• Partial batch success (some assets can fail)
• Batch events with success/failure counts

Files to Modify

• src/contracts/oracle_manager.rs -- add batch update
• src/contracts/multi_asset_oracle.rs -- batch processing
• tests/ -- batch update tests