Do not capture the SynchronizationContext in ConfigureAwait(ConfigureAwaitOptions)

[SimonCropp]

Bug

The Task.ConfigureAwait(ConfigureAwaitOptions) polyfill (and the Task<TResult> overload) implemented ForceYielding and SuppressThrowing with local async wrappers:

static async Task ForceYieldAsync(Task t) { await Task.Yield(); await t; }
static async Task SuppressThrowAsync(Task t) { try { await t; } catch { } }

Both wrappers await without ConfigureAwait(false), so they capture the current SynchronizationContext even when the caller did not pass ContinueOnCapturedContext. await task.ConfigureAwait(ConfigureAwaitOptions.ForceYielding) therefore deadlocks when the context thread is blocked, where the BCL completes. SuppressThrowing also paid for a throw and catch on every faulted or canceled task.

Fix

• ForceYielding: Task.Run(() => task), which always queues to the thread pool and unwraps to the original outcome.
• SuppressThrowing: a ContinueWith on TaskScheduler.Default with ExecuteSynchronously that reads Exception (marking it observed) and completes successfully. No exception is thrown.

ContinueOnCapturedContext is still honoured by the final ConfigureAwait(bool) on the resulting task.

Test

Added to PolyfillTests_Task_ConfigureAwait.cs: ForceYielding (both overloads) and SuppressThrowing awaited on a thread whose SynchronizationContext.Post never runs must complete within a timeout. These three timed out on net48 before the fix. Also added coverage for SuppressThrowing on a pending task and ForceYielding propagating a fault.