Enrich internal custom checks with flag showing them as such

[rbev]

This pull request introduces comprehensive improvements and new tests around the classification and reporting of internal custom checks, as well as enhancements to test coverage and code clarity. The main focus is ensuring that internal custom checks are correctly identified and exposed via the API, and that both primary and audit instances are consistent in their handling and registry of these checks. It also includes some test and project file cleanup to support these goals.

Custom check classification and reporting improvements:

• Added new acceptance tests to verify that internal custom checks are correctly flagged as internal, both for primary and persister checks, and that the API wire output reflects this (When_custom_checks_are_classified.cs, When_a_persister_check_fails.cs, When_the_body_storage_check_is_reported.cs). [1] [2] [3]
• Updated documentation to clarify the rules and guarantees around internal custom check versioning and classification (docs/data-versioning-design.md).

Audit and persistence test improvements:

• Added approval tests and included InternalCustomCheckClassification in audit and persistence test projects to ensure all custom checks are properly classified and registered, surfacing any missing entries (CustomChecksTest.cs, .csproj updates, approval files). [1] [2] [3] [4] [5] [6] [7]

Code and test cleanup:

• Fixed and clarified type usages in monitoring custom check tests to use the correct CustomCheckView and CustomCheck types, improving code clarity and correctness. [1] [2] [3]
• Removed unnecessary test files for features not supported by RavenDB (body storage custom check), and ensured only relevant tests are included in each test project.

Persistence API consistency:

• Changed the EF Core persistence implementation to return CustomCheckView instead of CustomCheck for stats queries, aligning with API expectations. [1] [2] [3]