Application fails to start when setting custom ApplicationName

> @captainsafia , @gfoidl , please have a closer look at the problem @Toudahl is complaining.
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> Here is the problematic part in `Microsoft.Extensions.DependencyInjection.MvcCoreServiceCollectionExtensions`
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> **The fact that the value is set by default to the entry-point assembly name, doesn't allow to treat the `ApplicationName` as assembly name and try loading it !!!**
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> This is breaking change that is not even documented.
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> Also, even though the `manager` instance could be provided via `DI`, the `PopulateDefaultParts` method is **internal** !!! and not marked as `override` - I hardly understand the reason behind letting user to injects their own implementation if he can't provide his own implementation of `PopulateDefaultParts` method.
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> Please reopen and provide estimations for the fix.
> Thanks

_Originally posted by @jvmlet in [#53791](https://github.com/dotnet/aspnetcore/issues/53791#issuecomment-2401780142)_