Add tests for payout mode immutability after market creation

[hman38705]

Area: contracts/predict-iq/src/modules/disputes.rs / market model

Problem: Comment says payout_mode should be immutable, but runtime behavior should be enforced by tests.

Testing gap: No test locking mode across resolve/dispute flows.

Acceptance tests: Verify resolve path cannot mutate payout mode unless explicitly allowed by spec.

[Kingsman-99]

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