Run mutation tests for any new tests added by each PR in CI

> Mutation Testing could cover such issues early ... e.g. if we only run mutation tests on the changes per PR, and only via the test cases that cover the `/src/` changes of the PR.
>
>

_Originally posted by @Flash0ver in [#5337](https://github.com/getsentry/sentry-dotnet/pull/5337/changes#r3520558060)_

### References

- [Microsoft docs - Mutation Testing](https://learn.microsoft.com/en-us/dotnet/core/testing/mutation-testing#adding-mutation-testing-to-your-cicd-workflow)
- [Stryker Mutator - Introduction](https://stryker-mutator.io/docs/stryker-net/introduction/)

Start by reviewing the repository's CI configuration and test layout, then read the Microsoft mutation-testing and Stryker.NET references linked in the issue. Determine how a PR's changed /src/ code and newly added tests can be scoped for mutation testing. Done means CI runs this coverage for applicable PRs and reports failures clearly.

Written by the indexing model from the issue text.