[browser][MT] .NET 9 regression of sync interop

[elringus]

Description

In .NET 8 it was possible to use sync/blocking interop in MT mode without any restrictions. For example, here is a sample which uses threading and nested sync interop for both JSImport and JSExport and works fine in .NET 8: https://github.com/elringus/bootsharp/tree/v0.3.3/samples/react

This same use case is no longer possible in .NET 9 (https://github.com/elringus/bootsharp/tree/main/samples/react)

Even with:

dotnet.withConfig({
    jsThreadBlockingMode: "DangerousAllowBlockingWait"
});

— it still throws when attempting to invoke sync JS function from within a sync JSExport.

Given this worked fine in .NET 8, it appears to be a regression. I'm fine with any kind of opt-in "dangerous" switches and "use at your own risk" contexts, but please restore the functionality .NET 8 had.

Reproduction Steps

1. Clone https://github.com/elringus/bootsharp/tree/main/samples/react
2. Follow instructions on https://github.com/elringus/bootsharp/blob/main/samples/react/README.md
3. Observe the errors about sync interop
4. Now clone the same repo, but under v0.3.3 tag, which runs on .NET 8
5. Follow same instructions
6. Observe no errors

Expected behavior

The sample should work on both .NET 8 and .NET 9

Actual behavior

The sample throw errors on .NET 9

Regression?

This is a regression in .NET 9

Known Workarounds

None.

Configuration

No response

Other information

No response

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