Prevent debugger inspection from initializing argument arity

[zjdangge]

Inspecting an argument in the debugger can initialize its cached arity before the argument has been added to a command or given a default value. Later parsing then uses the premature value and can report a missing required argument.

Add a debugger proxy that computes the displayed arity without caching it. Explicitly configured and already initialized arities are displayed as stored, while the normal Arity getter retains its existing behavior. The proxy also exposes the argument's metadata and the existing validator and completion collections without initializing them.

Fixes #2704.

Validation: the two regression cases for inspection before configuration failed before the fix. All 14 argument tests now pass, including explicit and previously initialized arity cases. The tests instantiate the proxy selected by DebuggerTypeProxyAttribute and verify subsequent parsing; no manual IDE debugger session was performed. A Windows Debug solution build completed with no warnings or errors, and all five test assemblies passed (1,949 passed, 14 existing skips), including the main tests on .NET 10 and .NET Framework 4.7.2. Release, Linux, and macOS were not run locally.