Duplicate routing rules

It should not be possible to create routing rules with the same priority, same audience, and same target for different payment providers. The Dashboard currently permits this.

Two rules saved with identical priority, target country and webshop setting but different providers both go live. The routing rule documentation recommends distinct priorities so developers "don't need to rely on tie-breaking behavior", but that tie-breaking behavior is not specified anywhere, so which provider a matching player is offered is unpredictable from the developer's side.

Reproduction steps

1. In the Unity Dashboard, open In App Purchases > Payment Providers, with at least two providers connected.
2. Under Provider routing, select Add routing rule. Set a priority, select a target country, enable Apply to webshops, and order the providers with provider A primary. Save.
3. Add a second routing rule with the same priority, the same target country and the same Apply to webshops setting, but with provider B primary. Save.

Expected behaviour

The second rule is rejected, with an error identifying the rule it conflicts with.

Actual behavior

The second rule saves. Rules activate immediately on save, so the ambiguous configuration goes live with no warning.

Notes

Applies to the Unity Dashboard payment provider routing configuration, not the SDK. D2C payment provider routing was introduced in IAP 5.4.0. Routing rules are evaluated server-side at runtime, so no client update is involved.