feat: Add validator for benchmarks that contains null runtime

[filzrev]

This PR add validator to handle issue that is reported #2609.

What's changed in this PR

1. Add RuntimeValidator.cs for benchmarks that have multiple runtimes and contains null.
2. Add RuntimeValidator to DefaultConfig's validator
3. Add unit tests.

Background
When benchmark don't contains Runtime characteristic. (e.g. When specify WithToolChain() only)
BenchmarkCase::GetRuntime returns current runtime instead of actual runtime that used by benchmark.

Normally, Runtime column is hidden on table because all benchmarks have same runtime value.
So it's not a problem. But when mixed WithRuntime settings. it display wrong runtime.

So it need to add validator to check benchmarks that contains multiple runtimes and contains null.

RuntimeValidator's error message

// * There are benchmarks that job don't have a Runtime characteristic. It's recommended explicitly specify runtime by WithRuntime

[timcassell]

I think the UX needs to be improved around this, but this is ok as a stop-gap for now.

[timcassell]

See comment

[filzrev]

I've updated runtime validation logics (457330d)
Additionally, I've added JobId (or Job text representation) to validation error message.

Example validation error messages

// Validating benchmarks:
//    * Job(Toolchain=.NET 10.0) don't have a Runtime characteristic. It's recommended to specify runtime by using WithRuntime explicitly.

//    * Job(Dry) don't have a Runtime characteristic. It's recommended to specify runtime by using WithRuntime explicitly.

//    * Job(abc) don't have a Runtime characteristic. It's recommended to specify runtime by using WithRuntime explicitly.

[filzrev]

It's not related to this PR thought. AllocationQuantumIsNotAnIssueForNetCore21Plus test seems flaky on CI.
Currently it's confirmed on mac-os (ARM64).

2025-06-15T04:21:35.2938130Z [xUnit.net 00:09:12.36]     AllocationQuantumIsNotAnIssueForNetCore21Plus(toolchain: .NET 8.0) [FAIL]
2025-06-15T04:21:35.2938900Z [xUnit.net 00:09:12.36]       Assert.Equal() Failure: Values differ
2025-06-15T04:21:35.2939300Z [xUnit.net 00:09:12.36]       Expected: 88
2025-06-15T04:21:35.2939630Z [xUnit.net 00:09:12.36]       Actual:   760

[timcassell]

Just thought about in-process toolchains after I merged. The current runtime behavior is correct on those toolchains, so they should not be included as part of the validation. @filzrev