MA0050: improve Throw* argument validation detection

[meziantou]

Why

MA0050 should recognize modern argument-validation helpers used before iterator execution. The previous check matched any method name containing Throw, which was broader than needed and could match unrelated method names.

What changed

• Updated MA0050 invocation detection to require static methods on ArgumentException (or derived types) whose names start with Throw.
• Added test coverage for:
  • ArgumentException.ThrowIfNullOrEmpty
  • custom ArgumentException-derived ThrowIf(...)
  • custom ArgumentException-derived Throw(...)
• Updated MA0050 rule documentation to describe support for static Throw* helpers on ArgumentException-derived types.

Notes

This keeps support for ArgumentNullException.ThrowIfNull and related helpers, while avoiding the old Contains("Throw") behavior.

Validation

dotnet CLI is not available in this environment, so tests and doc generation could not be executed locally in this session.