Migrate System.CommandLine 2.0-beta4 to 3.0-preview

[einari]

Summary

The daily automated Update Packages workflow has been failing every day trying to bump System.CommandLine from 2.0.0-beta4.22272.1 to the 3.0.0-preview line in Directory.Packages.props. That's a breaking API rewrite between the 2.x beta line and the 3.x preview line, so the bump alone (with no code changes) fails dotnet build with ~24 compiler errors in Source/Generator/Program.cs (CS1739, CS0117, CS1061, ...). Because the workflow only commits on a successful build, nothing was ever pushed to main — main has stayed safely on 2.0.0-beta4 while the workflow silently failed day after day.

This PR does the migration deliberately:

• Bumps System.CommandLine in Directory.Packages.props to 3.0.0-preview.7.26381.103 (the latest 3.0.0-preview build currently on NuGet.org, confirmed via dotnet package search System.CommandLine --prerelease).
• Ports Source/Generator/Program.cs (the only consumer of the API, shared into Source/Generator.Build via a source-file glob include) to the reworked 3.0-preview surface, preserving the exact same CLI options/behavior:
  • Option<T> construction no longer accepts a description: named parameter. Description is now a plain settable property (inherited from the Symbol base class), and IsRequired was renamed to Required.
  • getDefaultValue: is replaced by the DefaultValueFactory property (Func<ArgumentResult, T>).
  • RootCommand.AddOption(...) is replaced by the Options.Add(...) collection property.
  • Command.SetHandler(...) is replaced by Command.SetAction(...), which hands the action a ParseResult (+ CancellationToken for the async overload) instead of pre-bound option values — values are now pulled out explicitly via parseResult.GetValue(option).
  • RootCommand.InvokeAsync(args) is replaced by a two-step rootCommand.Parse(args) → parseResult.InvokeAsync().

The exact API shape was confirmed by decompiling the installed 3.0.0-preview.7.26381.103 package (via ilspycmd) rather than guessing from memory, since the 3.0-preview surface has moved across preview builds.

Test plan

• dotnet build at the repo root succeeds with zero errors (previously ~24 CS**** errors across Generator.csproj and Generator.Build.csproj).
• No test/Specs project exists in this solution, so none to run.
• Manual smoke test of the built protocol-generator tool:
  • --help output shows the same options, required flags, defaults, and descriptions as before.
  • Running with no arguments produces the same "Option '--assembly' is required." / "Option '--output' is required." errors + help text + exit code 1.
  • Running against a nonexistent assembly path exercises the existing HandleCommand catch block, printing Error: Assembly not found: ... and exiting 1 (same as before).
  • A full run against a real built assembly (Backend.dll) with default --base-namespace/--skip-segments successfully generates the expected service interfaces and DTOs.

🤖 Generated with Claude Code