HAMSTR-819 : Escrow: Limit number of open (active) arbitration cases

[HQS-Gun]

Motivation

In the new arbitration logic, the lifecycle of arbitration goes like this:

Proposal
A buyer or seller submits a proposal to either refund (to payer) or release (to receiver) a specific amount. At that point, it’s just a proposal.

Voting
Arbiters (those who are registered as arbiters for the escrow) vote on the proposal. When it reaches a certain number of ‘yes’ votes, it’s automatically set to “ACCEPTED” status. But it isn’t necessarily executed at that point.

Execution
Anyone can execute a proposal, IF it’s been accepted (if it’s status is ACCEPTED, that is, and if it still meets other criteria of still being relevant).

Currently, there is no limit on the number of arbitration cases that can be opened at once. There should be a limit, defined as a constant in ArbitrationModule.sol.

Changes:

1. Added MAX_ARBITRATION_CASES = 10 constant to limit concurrent arbitration cases
2. Added activeProposalCount tracking for active proposals
3. Implemented limit check in proposeArbitration() to enforce max cases
4. Fixed proposalCount increment for unique proposal ID generation
5. Added counter management for all proposal state transitions (EXECUTED, REJECTED, CANCELED)