Add StandardInputHandle/OutputHandle/ErrorHandle SafeFileHandle properties and LeaveHandlesOpen to ProcessStartInfo

[Copilot]

Description

Adds three new SafeFileHandle? properties and a LeaveHandlesOpen property to ProcessStartInfo that allow passing file handles directly to child processes, bypassing the RedirectStandard* pipe-based mechanism.

var psi = new ProcessStartInfo("grep") { ArgumentList = { "test" } };
SafeFileHandle.CreateAnonymousPipe(out SafeFileHandle readPipe, out SafeFileHandle writePipe);
psi.StandardInputHandle = readPipe;
psi.StandardOutputHandle = File.OpenHandle("output.txt", FileMode.Create, FileAccess.Write);
Process.Start(psi);

By default, Process.Start disposes the standard handles after starting the child process. Set LeaveHandlesOpen to true to keep them open for the caller to manage.

Changes

• ProcessStartInfo.cs: New StandardInputHandle, StandardOutputHandle, StandardErrorHandle auto-properties (SafeFileHandle?) with XML docs. Remarks document that handles do not need to be inheritable (the runtime duplicates them as inheritable), and list typical APIs (SafeFileHandle.CreateAnonymousPipe, File.OpenHandle, File.OpenNullHandle, Console.OpenStandardInputHandle/OpenStandardOutputHandle/OpenStandardErrorHandle), interaction with LeaveHandlesOpen, and constraints. New LeaveHandlesOpen property (bool, default false) controlling whether handles are disposed after process start.
• ref/System.Diagnostics.Process.cs: Public API surface additions for StandardInputHandle, StandardOutputHandle, StandardErrorHandle, and LeaveHandlesOpen
• Process.cs validation: When any Standard*Handle is set, UseShellExecute must be false and the corresponding RedirectStandard* must also be false. Reuses existing SR.CantRedirectStreams error message for the UseShellExecute check. A ValidateHandle helper method checks each provided handle for IsClosed (throws ObjectDisposedException) and IsInvalid (throws ArgumentException), following the same pattern as FileStream.ValidateHandle.
• Process.cs pipe creation: New handles take priority over default stdin/stdout/stderr inheritance. Handles are always disposed after starting the child process unless LeaveHandlesOpen is true.
• Strings.resx: New error strings CantSetHandleAndRedirect and Arg_InvalidHandle
• ProcessHandlesTests.cs: Tests for pipe redirection, inherited handles, piping, LeaveHandlesOpen behavior with File.OpenHandle, and validation error cases. Includes a LeaveHandlesOpen_KeepsHandleOpen test that verifies handles remain open after Process.Start when LeaveHandlesOpen is true, using hostname as a cross-platform command and File.OpenHandle for the output file.

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[adamsitnik]

@copilot please apply my suggestions and add one more test that uses File.OpenHandle to open a file handle for writing, set it via StandardOutput property and set LeaveHandlesOpen to true and verify that after starting the process the handle is still opened. Please choose command line utility that is available on every OS (hostname? whoami? pwd?)

[Copilot]

@copilot please apply my suggestions and add one more test that uses File.OpenHandle to open a file handle for writing, set it via StandardOutput property and set LeaveHandlesOpen to true and...

Applied both suggestions and added the LeaveHandlesOpen_KeepsHandleOpen test in 017f286. The test uses hostname (available on all OSes), opens a file via File.OpenHandle, sets LeaveHandlesOpen = true, and asserts the handle is still open after Process.Start.

[jkotas]

Is "as needed" correct? The implementation in the other PR is duplicating it as inheritable unconditionally.

[adamsitnik]

Not yet, that is why I want to introduce SafeFileHandle.IsInheritable in #125838.

[Copilot]

Updated wording to "the runtime will duplicate it as inheritable" in 7920b52.

[adamsitnik]

@copilot please address my feedback

[adamsitnik]

Not yet, that is why I want to introduce SafeFileHandle.IsInheritable in #125838.

[Copilot]

@copilot please address my feedback

All feedback addressed in 7920b52: renamed properties to *Handle, fixed "discard input" → "provide an empty input", added default-behavior note for null, changed "close" → "dispose" in docs, updated inheritability wording, and added ValidateHandle helper for invalid/closed checks.