Fix Post on disposed mailbox (#17849)

[majocha]

Fixes #17849

pulse disposal is under syncRoot lock now. Nullifying it ensures there is no attempt to set the disposed wait handle.

The overall result is that Post on disposed mailbox does not throw unless the mailbox was created with isThrowExceptionAfterDisposed = true. (See #13036)

The issue has minimal impact as certain rare conditions must be fulfilled to trigger the bug (#17849 (comment)).
This fix does not change the general behavior which is that a disposed mailbox should stop processing messages and (I presume) should not throw unless isThrowExceptionAfterDisposed is set.

[github-actions]

❗ Release notes required

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✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
src/FSharp.Core	docs/release-notes/.FSharp.Core/9.0.200.md

[psfinaki]

Do I get it right that this fixes this bug? If so, I guess the minimal repro your have there can serve as a test?

[psfinaki]

Thanks a lot!