Add Process.TryGetProcessById and silent param on Run/RunAsync

[SimonCropp]

Summary

Brings the Process polyfill in line with the .NET 11 BCL surface:

• Adds silent overloads to Process.Run / Process.RunAsync (suppress child stdout/stderr).
• Adds Process.TryGetProcessById(int, out Process?).

⚠️ Why this is a breaking change

Run and RunAsync gained a bool silent parameter, and to match the real .NET 11 signatures it was inserted in the middle of the parameter list — not appended at the end:

- Run(string fileName, IList<string>? arguments = null, TimeSpan? timeout = default)
+ Run(string fileName, IList<string>? arguments = null, bool silent = false, TimeSpan? timeout = default)

- RunAsync(string fileName, IList<string>? arguments = null, CancellationToken cancellationToken = default)
+ RunAsync(string fileName, IList<string>? arguments = null, bool silent = false, CancellationToken cancellationToken = default)

Because the new parameter sits in the 3rd positional slot (ahead of timeout / cancellationToken), any caller that passed the third argument positionally no longer compiles:

// Previously valid — now a compile error (CS1503):
Process.Run("dotnet", args, timeout);                 // TimeSpan? → bool
await Process.RunAsync("dotnet", args, cancelToken);  // CancellationToken → bool

Polyfill ships as source, so this is a source/compile-time break that surfaces the moment a consumer updates the package and rebuilds (there's no binary-compat layer to absorb it). All existing Run/RunAsync call sites using a positional 3rd argument must be updated.

Why insert mid-list instead of appending?

The parameter order deliberately mirrors the .NET 11 BCL:

• Process.Run(string, IList<string>, bool, TimeSpan?)
• Process.RunAsync(string, IList<string>, bool, CancellationToken)

Appending silent at the end would keep source-compat today but diverge from the BCL, so the same call would break later when a consumer moves to net11 and the polyfill retires in favor of the framework method (ProcessPolyfill.cs is gated #if !NET11_0_OR_GREATER). Taking the one-time break now keeps call sites identical across the polyfill→BCL boundary.

Migration

Use named arguments for the trailing parameter:

- Process.Run("dotnet", args, timeout);
+ Process.Run("dotnet", args, timeout: timeout);

- await Process.RunAsync("dotnet", args, cancelToken);
+ await Process.RunAsync("dotnet", args, cancellationToken: cancelToken);

Or pass silent explicitly if adopting the new behavior.

Non-breaking additions

• Process.TryGetProcessById(int processId, out Process? process) — purely additive (throws ArgumentOutOfRangeException for processId <= 0, returns false when no such process exists).
• silent: true on Run/RunAsync redirects and discards stdout/stderr. Note: on pre-net11 the polyfill suppresses by redirecting-and-discarding (stdin stays connected), which differs slightly from net11 binding to the null device.