Ensure IServiceBroker is used on a background thread

[sharwell]

Expands #47903 to other calls to GetProxyAsync.

[sharwell]

@tmat @CyrusNajmabadi @jinujoseph roslyn-integration-CI caught this error in the OOP testing legs, but those legs have been disabled so no one saw it.

[jasonmalinowski]

@sharwell What's the background here as far as why this is necessary? Do we have a writeup for the team guidance here?

[sharwell]

@jasonmalinowski See #47903 for background. This is the same issue.

[jasonmalinowski]

@sharwell, I did, hence my request for an explanation. 😄

[sharwell]

The official guidance is to follow the three threading rules. Code which does so will avoid runtime deadlocks. Roslyn intentionally does not follow these rules, opting instead to mitigate observed deadlocks on an as-needed basis using different strategies based on the type of thread dependency that led to the deadlock. The strategy employed here is the manual mitigation strategy for a UI thread dependency introduced by external code using ConfigureAwait(true) (explicitly or implicitly).