feat(cli): add public completion parsing API for external autocompletion

[JKamsker]

This PR adds a tiny, additive public surface to Spectre.Console.Cli so third-party autocompletion can work without reflection or internals access.

Why

External projects (e.g. my autocompletion lib on branch fix/autocomplete-public-api) previously had to use reflection/internal types to:

• Determine the current command/leaf context from a partial command line
• Access the app’s DI resolver to run dynamic suggestions

What’s included (surgical + additive)

• New public Spectre.Console.Cli.ITypeResolverAccessor to expose the current ITypeResolver during execution
• New public Spectre.Console.Cli.Completion.CommandLineParser + CommandLineParseResult + CommandLineMappedParameter to parse args against the command model and return:
  • leaf Help.ICommandInfo?
  • CommandType / SettingsType
  • mapped ICommandParameterInfo + value pairs
  • remaining arguments
• Minimal wiring in Internal/CommandExecutor.cs to register:
  • ICommandAppSettings
  • Help.ICommandModel
  • ITypeResolverAccessor (populated for the lifetime of the resolver)

Consumer reference

JKToolKit autocompletion refactor using only public contracts

Notes

• No breaking changes; no behavioral changes for normal command execution
• The new completion API is a thin wrapper over existing internal parsing behavior

[patriksvensson]

@JKamsker Looks good! Would need to have some test coverage, though, before we can get this merged.

[JKamsker]

Thanks for the quick reply @patriksvensson ! I have added some tests (i hope they suffice 😅 ) and i am confident the pr is ready for review

[JKamsker]

Hey @patriksvensson is there anything i can do to get this across the finishing line?

[patriksvensson]

@JKamsker I haven't got around to review this yet. Will try to do it in the upcoming week. Sorry about this.

[JKamsker]

Thank you for the response, @patriksvensson!

As a side note: I'm not entirely convinced that registering ICommandModel, ICommandAppSettings, and ITypeResolverAccessor into the DI container is the cleanest way to enable external autocompletion, but I haven't been able to come up with a better alternative. If you have any ideas for a different design, I'd really appreciate the input!