Add Process.StartAndForget APIs for fire-and-forget process launching

[Copilot]

Adds two new static methods to Process for scenarios where you want to launch a process, get its PID, and immediately release all resources without waiting for it to exit. API approved in #125838 (comment).

Description

New APIs (Process.Scenarios.cs):

• Process.StartAndForget(ProcessStartInfo) — validates that UseShellExecute is not set (throws InvalidOperationException if UseShellExecute = true, since on Windows shell execution may not create a new process, making it impossible to return a valid PID) and that no stream redirection is configured (throws InvalidOperationException if any RedirectStandardInput/Output/Error flag is set). Starts the process via SafeProcessHandle.Start, releases all associated resources, and returns the PID. When no standard handles are provided via ProcessStartInfo.StandardInputHandle/StandardOutputHandle/StandardErrorHandle, they are redirected to the null device by default. When possible, handle inheritance is limited to the standard handles only.
• Process.StartAndForget(string fileName, IList<string>? arguments = null) — builds a ProcessStartInfo and delegates to the ProcessStartInfo overload above. No handles are inherited by the new process.

// Before: resource leak unless caller explicitly disposes
Process p = Process.Start("myapp", args);
int pid = p.Id;
p.Dispose(); // easy to forget

// After: idiomatic, guaranteed cleanup
int pid = Process.StartAndForget("myapp", args);

Validation guards — throws InvalidOperationException if:

• UseShellExecute = true (shell execution may not start a new process on Windows, making it impossible to return a valid PID)
• Any RedirectStandardInput/Output/Error flag is set (use ProcessStartInfo.StandardInputHandle = File.OpenNullHandle() instead to redirect to null explicitly)

Supporting changes:

• Uses SafeProcessHandle.Start (introduced in PR Introduce SafeProcessHandle.Start and ProcessId #126192) directly, avoiding the need to create and dispose a Process instance
• Reference assembly updated with both signatures, including the // this needs to come after the ios attribute due to limitations in the platform analyzer comment on [SupportedOSPlatformAttribute("maccatalyst")], consistent with other Start overloads in the file
• New string resource StartAndForget_UseShellExecuteNotSupported in Strings.resx; the redirect-not-supported error message (CantSetRedirectForSafeProcessHandleStart) mentions both SafeProcessHandle.Start and Process.StartAndForget so users calling either API get a clear, actionable error: "The RedirectStandardInput, RedirectStandardOutput, and RedirectStandardError properties cannot be used by SafeProcessHandle.Start or Process.StartAndForget. Use the StandardInputHandle, StandardOutputHandle, and StandardErrorHandle properties."
• The bool parameter on SafeProcessHandle.Start(ProcessStartInfo, bool) renamed from redirectToNull to fallbackToNull for clarity
• Unused using System.IO and using System.Runtime.InteropServices directives removed from Process.Scenarios.cs
• <summary> XML docs on both StartAndForget overloads clearly describe the fire-and-forget semantics (starts the process, releases resources, and returns the PID)
• <remarks> paragraphs in StartAndForget(ProcessStartInfo) reordered: the standard-handle redirect paragraph is first, followed by the handle-inheritance paragraph
• Process.Scenarios.cs added to System.Diagnostics.Process.csproj
• Tests in StartAndForget.cs:
  • Happy paths (both overloads) in a single [ConditionalTheory] parameterized by bool useProcessStartInfo, using CreateSleepProcess with a 1-hour timeout and using declaration, with explicit Kill()/WaitForExit() cleanup in try/finally
  • StartAndForget_WithStandardOutputHandle_CapturesOutput uses the CreateProcess/RemoteExecutor pattern with SafeFileHandle.CreateAnonymousPipe to verify that ArgumentList is not ignored and that StandardOutputHandle correctly captures process output — mirrors the CanRedirectOutputToPipe pattern from ProcessHandlesTests
  • ArgumentNullException tests use AssertExtensions.Throws to verify parameter names ("startInfo" and "fileName")
  • InvalidOperationException for each redirect flag combination and for UseShellExecute = true via [Theory]
  • Null-arguments test uses cmd.exe on Windows (available on Nano Server) and true on Unix

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[adamsitnik]

@copilot address my feedback

[adamsitnik]

@copilot address my feedback

[adamsitnik]

@copilot address my feedback

[danmoseley]

i only see API approval issue for public static int StartAndForget(ProcessStartInfo startInfo); not the other one?

[adamsitnik]

i only see API approval issue for public static int StartAndForget(ProcessStartInfo startInfo); not the other one?

Please check #125838 (comment)

@copilot please add this exact link to the PR description. And update the description based on the current state of the PR

[danmoseley]

i only see API approval issue for public static int StartAndForget(ProcessStartInfo startInfo); not the other one?

Please check #125838 (comment)

@copilot please add this exact link to the PR description. And update the description based on the current state of the PR

Ah I thought api review always update the top post to final shape. ok

[Copilot]

i only see API approval issue for public static int StartAndForget(ProcessStartInfo startInfo); not the other one?

Please check #125838 (comment)...

Added the link #125838 (comment) to the PR description and updated it to reflect the current state of the PR.