Add ProcessStartOptions class with platform-aware path resolution

[Copilot]

Description

Adds ProcessStartOptions class providing explicit path resolution and process configuration with platform-specific semantics.

Changes

API Surface

• ProcessStartOptions(string fileName) - resolves executable path and validates existence
• Properties: FileName, Arguments, Environment, WorkingDirectory, InheritedHandles, KillOnParentExit, CreateNewProcessGroup
• Arguments, Environment, InheritedHandles use lazy allocation
• Environment populated from Environment.GetEnvironmentVariables() using Dictionary<string, string?>
• Null validation on Arguments and InheritedHandles setters
• Constructor throws FileNotFoundException with localized message when path cannot be resolved or file doesn't exist

Path Resolution

• Windows: appends .exe (except when path contains separators, ends with ., or has extension) → System32 → PATH (using ; separator)
• Unix: PATH only (using : separator)
• Current directory accessed only with explicit ./ (Unix) or .\ (Windows) prefix
• Returns rooted, fully-qualified paths via Path.GetFullPath for non-fully-qualified inputs

Implementation Structure

• ProcessUtils partial class consolidates shared logic:
  • ProcessUtils.cs - shared FindProgramInPath with comment explaining Unix permission checking
  • ProcessUtils.Windows.cs - private IsExecutable (forwards to File.Exists)
  • ProcessUtils.Unix.cs - private IsExecutable (validates executable permissions)
• Process.Unix.cs calls ProcessStartOptions.ResolvePath (eliminates duplicate Process.ResolvePath)
• Process.IsExecutable made internal for reuse
• Uses Environment.SystemDirectory instead of duplicating GetSystemDirectoryW
• Caches Windows directory in s_cachedWindowsDirectory
• Localized error string SR.FileNotFoundResolvePath

Tests

• 25 tests across 3 partial class files (base, Windows, Unix)
• Target-typed new() syntax, [Theory] with [InlineData] for parameterized tests
• Platform-specific assertions: EndsWith for paths (handles symlink variations), Path.GetFullPath for macOS /tmp symlinks
• Conditional attributes: IsNotWindowsNanoServer, IsNotWindowsServerCore
• Tests validate: lazy initialization, path resolution, platform separators, error cases

Customer Impact

Provides safer, more explicit alternative to ProcessStartInfo for scenarios requiring validated executable paths and controlled environment inheritance.

Testing

• Manual: Library builds successfully
• Automated: 25 unit tests covering path resolution, lazy initialization, platform-specific behavior, error cases

Example

// Resolves and validates path during construction
var options = new ProcessStartOptions("cmd");
Console.WriteLine(options.FileName); // C:\Windows\System32\cmd.exe

// Lazy-allocated collections
options.Arguments.Add("/c");
options.Arguments.Add("echo test");

// Environment inherits current process, modifiable
options.Environment["MY_VAR"] = "value";

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-diagnostics-process
See info in area-owners.md if you want to be subscribed.

[adamsitnik]

@copilot please address my feedback

[adamsitnik]

@copilot address my feedback

[adamsitnik]

@copilot please address my feedback

[adamsitnik]

@copilot address my feedback

[jkotas]

@copilot Address all feedback that was not addressed yet

[jkotas]

LGTM!

[adamsitnik]

/ba-g all build and test failures are known