IDE Threading improvements

[sharwell]

• Use IThreadingContext instead of ThreadHelper

Currently we use both IThreadingContext and ThreadHelper. We should unify code on the former for two reasons:

1. Consistency
2. IThreadingContext allows for work tracking, which improves the ability to isolate tests that run asynchronous code

❓ Is there a reason this isn't using IThreadingContext? It seems like we should be using that interface to the extent possible.

Originally posted by @sharwell in #30451

• Audit uses of ContinueWith. Audit uses of ContinueWith, SafeContinueWith, StartNew, SafeStartNew, and Task.Run to identify cases where simpler approaches are viable
• Uniformly use a main thread check/assert method. Design, implement, and uniformly use a main thread check/assert method
• Configure problematic synchronous waits. Configure WaitAndGetResult and WaitAndGetResult_CanCallOnBackground as problematic synchronous waits (blocked on Allow extensions for VSTHRD002 (Avoid problematic synchronous waits) microsoft/vs-threading#344)
• Review code for additional methods to consider legacy thread switching APIs. From Migrate to Microsoft.VisualStudio.Threading #28781. This is blocked on Allow extensions for VSTHRD001 (Avoid legacy thread switching APIs) microsoft/vs-threading#332, but once available we should review the code for thread-switching APIs and configure them so they are recognized by analyzers in vs-threading.
• Eliminate JoinableTaskFactoryTaskScheduler. From Migrate to Microsoft.VisualStudio.Threading #28781 (comment). Eliminate JoinableTaskFactoryTaskScheduler, or at least add it to the set of legacy thread switching types. Related to Review code for additional methods to consider legacy thread switching APIs #29277

[CyrusNajmabadi]

Closing out as speculative.