Ensure BackgroundService invokes ExecuteAsync after start

[steveisok]

Fixes #131249.

BackgroundService.StartAsync passed its stopping token to Task.Run as the scheduling token. If StopAsync or Dispose canceled that token before the queued delegate began, the task transitioned to Canceled without invoking ExecuteAsync.

This change:

• Preserves the .NET 10 behavior where all ExecuteAsync work runs asynchronously on a thread-pool thread.
• Uses CancellationToken.None for scheduling so cancellation cannot suppress delegate invocation after a non-canceled start is accepted.
• Preserves pre-canceled StartAsync behavior by explicitly assigning Task.FromCanceled(cancellationToken) to ExecuteTask.

Regression tests deterministically occupy the sole thread-pool worker and verify that immediate stop and dispose still invoke ExecuteAsync exactly once on a thread-pool thread with an already-canceled stopping token. Pre-canceled startup coverage also verifies that ExecuteAsync is not invoked.

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[steveisok]

@jeffhandley deferring to you on who best to review.

[Copilot]

Pull request overview

This PR adjusts BackgroundService.StartAsync scheduling so that ExecuteAsync is still invoked even if the service is stopped/disposed immediately after start, and adds regression coverage to validate the behavior under deterministic thread-pool starvation.

Changes:

• Update BackgroundService.StartAsync to avoid using the stopping token as the Task.Run scheduling token.
• Add a regression test that blocks the sole thread-pool worker and verifies ExecuteAsync still runs exactly once when immediately stopped/disposed.
• Update the pre-canceled StartAsync test to assert ExecuteTask is canceled and ExecuteAsync is not invoked.

Show a summary per file

File	Description
src/libraries/Microsoft.Extensions.Hosting.Abstractions/src/BackgroundService.cs	Changes how ExecuteAsync is scheduled/canceled to prevent cancellation from suppressing delegate invocation.
src/libraries/Microsoft.Extensions.Hosting/tests/UnitTests/BackgroundServiceTests.cs	Adds deterministic regression coverage for immediate stop/dispose and refines pre-canceled start assertions.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 2
• Review effort level: Lite