Unix process termination

[tmat]

Multi-targets DotNetDeltaApplier in order to use Posix signal registration API. Register SIGTERM signal and exit the process. This is only needed for net10+ due to dotnet/docs#46226.

Since our startup hook is the first to execute in the app process, the registered handler will be the last to execute. See dotnet/runtime#116557.

The change also had to update WaitForExitAsync to avoid hangs on Unix. It's unclear why. The implementation of this API is generally unreliable, see dotnet/runtime#109434.

Blazor and WebApp tests still disabled. Needs more investigation.

[dotnet-policy-service]

Thanks for your PR, @@tmat.
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[tmat]

@DustinCampbell ptal

[Copilot]

Pull Request Overview

This PR enhances Unix process termination and reliability for dotnet watch by multi-targeting the DeltaApplier, registering a SIGTERM handler, and improving WaitForExitAsync to avoid hangs. It also updates tests to be cross-platform and configures polling for file-watcher tests.

• Add SIGTERM registration in the startup hook for graceful shutdown on Unix (net10+).
• Refactor ProcessRunner.WaitForExitAsync with a polling loop to address hangs on Linux.
• Enable polling file watcher in tests and remove Windows‐only [PlatformSpecificFact] attributes.

Reviewed Changes

Copilot reviewed 33 out of 33 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
test/.../WatchableApp.cs	Add comment and enable polling file watcher in tests
test/.../ProgramTests.cs (and many)	Replace [PlatformSpecificFact/Theory] with [Fact/Theory] for cross-platform tests
test/.../TestOptions.cs	Use 1s timeout on Unix and 0s on Windows for process cleanup
test/TestAssets/.../AppWithDeps/Program.cs	Increase loop delay from 10ms to 1000ms in test asset
src/.../dotnet-watch.csproj	Multi-target Microsoft.Extensions.DotNetDeltaApplier for net10.0 and netstandard2.1
src/.../Internal/ProcessRunner.cs	Refactor WaitForExitAsync to poll HasExited and log verbose messages
src/.../HotReload/StartupHook.cs	Add RegisterPosixSignalHandlers to handle SIGTERM and exit process

Comments suppressed due to low confidence (1)

src/BuiltInTools/DotNetDeltaApplier/StartupHook.cs:86

• No tests cover the new SIGTERM handler. Consider adding integration or unit tests to verify that the handler correctly triggers Environment.Exit(0) on Unix when SIGTERM is received.

    private static void RegisterPosixSignalHandlers()