[Cohosting] `@bind-<prop>` tag helper descriptors don't get created

I've spent an inordinate amount of time trying to work out why generating an event handler for `@bind-Value:set` doesn't work in our cohosting tests and I've finally gotten to the bottom of it.
This test: https://github.com/dotnet/razor/blob/8babb3ad96fe6ac7a28619b18b2f3a0c352e3d1a/src/Razor/test/Microsoft.VisualStudio.LanguageServices.Razor.Test/Cohost/CodeActions/GenerateEventHandlerTests.cs#L59

In the regular tooling code we discover tag helpers by creating a `TagHelperDescriptorProviderContext` with no target symbol, here: https://github.com/dotnet/razor/blob/8babb3ad96fe6ac7a28619b18b2f3a0c352e3d1a/src/Razor/src/Microsoft.CodeAnalysis.Razor.Workspaces/Extensions/ProjectExtensions.cs#L55
In the source generator when finding tag helpers in references, the assembly symbol is used as the target symbol: https://github.com/dotnet/razor/blob/8babb3ad96fe6ac7a28619b18b2f3a0c352e3d1a/src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/SourceGenerators/RazorSourceGenerator.cs#L214

However, when the `BindTagHelperDescriptorProvider` runs, if the target symbol isn't null, it won't run unless the target symbol (ie assembly) is the same assembly as `Microsoft.AspNetCore.Components.BindConverter` is defined in. `BindConverter` is contained in `Microsoft.AspNetCore.Components.dll` and all of the standard components, like `InputText` etc., are in `Microsoft.AspNetCore.Components.Web.dll`.

Changing the source generator to pass `null` as the target symbol makes the test pass, but it also makes things take a _lot_ longer. Removing the logic in `BindTagHelperDescriptorProvider` also works, but there is a test that specifically validates the behaviour: https://github.com/dotnet/razor/blob/8babb3ad96fe6ac7a28619b18b2f3a0c352e3d1a/src/Compiler/Microsoft.CodeAnalysis.Razor/test/BindTagHelperDescriptorProviderTest.cs#L223

I'm guessing things work fine at runtime because the tag helpers are only in place to help tooling. I can't personally see any reason to keep the logic in `BindTagHelperDescriptorProvider` but maybe someone else has some more context. I removed it and checked that the generate methods tests pass, but I haven't run all of the rest of the compiler tests to know if there is any other impact.