feat(commands): support Task<TResult> as async execute return type #56

Summary

[DelegateCommand] / [AsyncDelegateCommand] currently reject execute methods that return Task<TResult> (documented in README and enforced in DelegateCommandGenerator).

Motivation

Some async APIs return Task<T> rather than non-generic Task or ValueTask<T>. Supporting this shape would reduce friction when wiring Prism commands.

Proposed direction

• Define emission strategy (e.g. discard result vs. surface as command generic type).
• Add diagnostics for unsupported combinations (similar to PSG1001 for CancellationToken + ValueTask).
• Extend unit/snapshot tests and README/docs.

References

• README: Task<TResult> is not supported for execute methods.
• DelegateCommandGenerator.cs comment: unchanged since initial release.