Detect and forbid mutation in validation methods

### Description

Service owners seem to think it is a good idea to mutate their data models in a `IFormDataValidator`. As our validators run in parallel, such mutations might cause race conditions. In addition it causes issues with our `PATCH` updates, when the mutations in the validator, does not follow the invariants defined in the `ObjectUtils` methods

https://github.com/Altinn/app-lib-dotnet/blob/1ac59d83211eae2d37ad8998cbc1c10c52c54632/src/Altinn.App.Core/Internal/Patch/PatchService.cs#L125-L149

### In scope

Pick a solution (or add another)
1. Find an (efficient - or dev only) way to detect and forbid mutations of the data object in validators.
2. Find an efficient way to always run validation on a copy to fix this particular issue.
3. Alternatively we can try to fix our code so that this (and other cases of validator mutations) work as expected by service owners

### Out of scope

_No response_

### Additional Information

_No response_

### Analysis

_No response_

### Conclusion

_No response_

Start with src/Altinn.App.Core/Internal/Patch/PatchService.cs around lines 125-149 and trace how IFormDataValidator instances receive and modify data models. The issue is complete when one agreed solution prevents validator mutations from causing parallel-validation races and PATCH invariant problems, with its behavior covered by the relevant tests.

Written by the indexing model from the issue text.