C#: Use safe pattern-match in CSharpVisitor for VisitExpression/VisitStatement (#7133)

* C#: Use safe pattern-match in CSharpVisitor for VisitExpression/VisitStatement

Replace unsafe direct casts with pattern-match early returns in all
CSharpVisitor methods that call VisitExpression() or VisitStatement(),
matching the established pattern in JavaVisitor.cs.

* C#: Add tests for safe VisitStatement/VisitExpression type replacement

Verify that when VisitStatement or VisitExpression returns a different
type, the visitor method returns that result gracefully instead of
throwing a ClassCastException.

* C#: Add JavaVisitor tests for safe VisitStatement/VisitExpression type replacement

Author: knutwannheden