fix(C#): Enhance number handling in SystemTextJsonCSharpRenderer

[mikecrowe-otp]

Description

• Added support for handling both integer and double types in a union case.
• Introduced a variable generator to manage variable naming dynamically.
• Updated decoder transformer cases to accommodate the new logic for number types.
• Ensured backward compatibility for existing single type handling.

Related Issue

Fixes #2804

Motivation and Context

C# code would not compile

Previous Behaviour / Output

error CS0152: The switch statement contains multiple cases with the label value '8'

New Behaviour / Output

Code compiles

How Has This Been Tested?

In our production code

Screenshots (if appropriate):