[CI Failure] Flaky: AppHostServerSessionTests.Start_StopRequested_GracefulSignalerThrows_StillEscalatesToKill fails intermittently on Windows

## Build Information

Build: https://github.com/microsoft/aspire/actions/runs/34815567672
Build error leg or test failing: Tests / No-package tests (regular, Aspire.Cli.Tests, Cli, Cli, tests/Aspire.Cli.Tests/Aspire.Cli.Tests.cs... / Cli (windows-latest) / `Aspire.Cli.Tests.Projects.AppHostServerSessionTests.Start_StopRequested_GracefulSignalerThrows_StillEscalatesToKill`
Pull request: #18763

## Classification Analysis

Timing-sensitive assertion on process escalation/kill sequence in a test file not touched by the PR; sibling tests in the same class have documented flakiness under CI load.

## Failure Information

Test output

Error:

Assert.Single() Failure: The collection was empty


Stack Trace:

at Aspire.Cli.Tests.Projects.AppHostServerSessionTests.Start_StopRequested_GracefulSignalerThrows_StillEscalatesToKill() in D:\a\aspire\aspire\tests\Aspire.Cli.Tests\Projects\AppHostServerSessionTests.cs:line 337

   at Aspire.Cli.Tests.Projects.AppHostServerSessionTests.Start_StopRequested_GracefulSignalerThrows_StillEscalatesToKill() in D:\a\aspire\aspire\tests\Aspire.Cli.Tests\Projects\AppHostServerSessionTests.cs:line 337

--- End of stack trace from previous location ---

## Description

Flaky: AppHostServerSessionTests.Start_StopRequested_GracefulSignalerThrows_StillEscalatesToKill fails intermittently on Windows

**Type**: flaky-test

## Occurrences

| Date | Build | Job | PR |
|------|-------|-----|----|
| 2026-09-14 | [34815567672](https://github.com/microsoft/aspire/actions/runs/34815567672) | Tests / No-package tests (regular, Aspire.Cli.Tests, Cli, Cli, tests/Aspire.Cli.Tests/Aspire.Cli.Tests.cs... / Cli (windows-latest) | #18763 |

Start in tests/Aspire.Cli.Tests/Projects/AppHostServerSessionTests.cs at line 337 and read the sibling tests in the same class for their documented CI flakiness. Run Start_StopRequested_GracefulSignalerThrows_StillEscalatesToKill on Windows, focusing on the timing-sensitive escalation assertion. Done means the test reliably observes the expected kill sequence under CI load.

Written by the indexing model from the issue text.