feat: per-buyer cap on concurrent open disputes

[shakurJJ]

Summary

set_max_concurrent_disputes caps concurrent disputes contract-wide. This proposes a separate, optional per-buyer cap so one high-dispute buyer can't consume the entire global budget.

Proposed Behaviour

• Admin can set a per-buyer concurrent open-dispute cap, independent of the global cap.
• raise_dispute/raise_partial_dispute check both the global cap (existing behaviour) and the buyer's cap (if configured) before allowing a new dispute.
• A buyer with no configured cap is subject only to the global cap, unchanged from today.

Acceptance Criteria

• Admin can set/get a buyer's concurrent-dispute cap
• A buyer with no configured cap is unaffected (today's behaviour)
• A buyer at their configured cap cannot raise a new dispute even if the global cap has room
• Tests cover both caps interacting

[Udokwuebube]

@Udokwuebube has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

hi, i would like to work on this.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Udokwuebube to this issue.

[TINEROW]

@TINEROW has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello Maintainer, I’ve reviewed the issue and understand the requirements. I’m confident I can implement an effective solution. Please consider assigning it to me.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @TINEROW to this issue.