Replace ProcessUtilities.SendPosixSignal with SafeProcessHandle.Signal API

[Copilot]

Migrate from the manual P/Invoke kill wrapper (ProcessUtilities.SendPosixSignal) to the new SafeProcessHandle.Signal(PosixSignal) API introduced in dotnet/runtime#126313.

Changes

• ProcessRunner.cs: TerminateUnixProcess now takes a Process and calls process.SafeHandle.Signal(PosixSignal.SIGKILL / SIGTERM). Annotated [UnsupportedOSPlatform("windows")] to satisfy CA1416.
• PhysicalConsole.cs: Resolves process by ID via Process.GetProcessById to obtain a handle for Signal. Catches ArgumentException for already-exited processes.
• ProcessUtilities.cs: Removed SendPosixSignal, SIGKILL, and SIGTERM constants. SendWindowsCtrlCEvent remains.

Before:

var error = ProcessUtilities.SendPosixSignal(state.ProcessId,
    signal: force ? ProcessUtilities.SIGKILL : ProcessUtilities.SIGTERM);

After:

process.SafeHandle.Signal(force ? PosixSignal.SIGKILL : PosixSignal.SIGTERM);

[adamsitnik]

@copilot please address my feedback

[Copilot]

Pull request overview

This PR migrates dotnet-watch’s Unix process termination from a custom kill P/Invoke (ProcessUtilities.SendPosixSignal) to the newer SafeProcessHandle.Signal(PosixSignal) API, aligning the implementation with the runtime-provided signaling mechanism.

Changes:

• Remove the SendPosixSignal P/Invoke wrapper and related SIG* constants from ProcessUtilities.
• Update ProcessRunner to signal the existing Process via process.SafeHandle.Signal(...) on Unix.
• Update PhysicalConsole to resolve processes by PID (Process.GetProcessById) in order to obtain a SafeHandle for signaling.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
src/Dotnet.Watch/Watch/Utilities/ProcessUtilities.cs	Removes the custom kill P/Invoke signal-sending helper and constants, leaving only the Windows Ctrl+C path.
src/Dotnet.Watch/Watch/UI/PhysicalConsole.cs	Uses Process.GetProcessById(...).SafeHandle.Signal(PosixSignal.SIGTERM) to propagate Ctrl+C/SIGTERM behavior to child processes on Unix.
src/Dotnet.Watch/Watch/Process/ProcessRunner.cs	Uses process.SafeHandle.Signal(...) for Unix termination, and adds platform annotation to satisfy platform compatibility analysis.