feat: add dynamic bid cancellation and collateral refunds

[Benedict315]

Solution Description: SC-REG-028 - Dynamic Bid Cancellation and Collateral Refund Flows
Implementation Overview
Implement a bid cancellation system in the Job Registry contract that allows bidders to cancel active bids and receive collateral refunds with proper ownership validation and state management.

Key Components
Data Structures:

Add BidCancellation metadata tracking cancellation state and refund status
Map Job/Bid IDs to cancellation data

Cancellation Logic:

Validate bidder ownership and bid status (only active bids)
Prevent cancellation after job assignment
Execute atomic collateral refund or fail safely
Update bid status to Cancelled
IPFS Optimization:

Store cancellation reasons as IPFS CIDs instead of strings
Reduces on-chain footprint
Error Codes:

UnauthorizedCancellation, InvalidBidStatus, CollateralRefundFailed, NoCollateralToRefund

Testing:

Happy path: cancel active bid → receive refund
Edge cases: late cancellation, double-cancel, access control
Target: >90% coverage

closes #382

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