RCS1202 Avoid NullRef - Needs to be more strict

**Steps to Reproduce**:
```csharp
interface Foo {
bool? Get();
}

class Bar {
void DoStuff(Foo foo) {
var x = foo.Get().Value;
}
}
```

**Actual Behavior**:
RCS1202 Avoid NullRef not triggered

**Expected Behavior**:
RCS1202 Avoid NullRef should be triggered

**Other**:
Think RCS1202 should be a warning by default.

Start from the RCS1202 analyzer and its existing tests, then reproduce the nullable bool .Value case shown in the issue. Done means the diagnostic is reported for that case; the separate request to change the default severity should be treated as a distinct decision.

Written by the indexing model from the issue text.