feat(dispute): add get_open_disputes view

[AbdulmujibOladayo]

Summary

Adds a global open disputes list so resolvers can discover which disputes await their action.

Changes

• Add DataKey::OpenDisputes to storage_types.rs
• open_dispute appends to the list; resolve_dispute removes from it
• Add get_open_disputes(e) -> Vec in dispute.rs
• Expose in contract.rs

closes #200

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