Fix NullReferenceException in math_change block with undeclared variables

[Copilot]

The math_change block throws when operating on undeclared or null variables, inconsistent with variables_set and text_append which auto-initialize.

Changes

• MathChange.cs: Add null-safe checks before casting, check root context for scope resolution, initialize undeclared/null variables to 0.0
• MathTests.cs: Add tests for undeclared and null variable cases

Example

Before:

if (context.Variables.ContainsKey(variableName))
{
  var value = (double)context.Variables[variableName];  // NullReferenceException if value is null
  // ...
}
else
{
  throw new ApplicationException($"variable {variableName} not declared");
}

After:

if (variables.ContainsKey(variableName) && variables[variableName] != null)
{
  value = (double)variables[variableName];
}
else
{
  var rootContext = context.GetRootContext();
  if (rootContext.Variables.ContainsKey(variableName) && rootContext.Variables[variableName] != null)
  {
    value = (double)rootContext.Variables[variableName];
    // Update in root context and return
  }
  else
  {
    value = 0.0;  // Initialize to default numeric value
  }
}

Handles both variables declared in XML but never initialized, and variables explicitly set to null by blocks like ControlsFor.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>ApplicationException when undeclared variable is used</issue_title>
<issue_description>The block "math_change" worked not as aspected.

Here is the error message:

Here is my Example xml:
TestResult.txt

I get the error message when i run your unit test with this formular, too.

Originally posted by @Amboryus in #53 (comment)</issue_description>

Comments on the Issue (you are @copilot in this section)

@richorama Thank you for posting this error. I believe this is because the 'TestResult' variable is not declared in your blockly program. This is a case that IronBlock currently doesn't handle. I will add support for it. @richorama hmm, I wonder if it's a scope problem - I'll continue to investigate...

• Fixes ApplicationException when undeclared variable is used #55

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