Add Formula Exception Stubs to System Namespace

[tmitchellcertinia]

Summary

This PR adds FormulaEvaluationException and FormulaValidationException to the System package so apex-ls resolves the exceptions the FormulaEval API throws, instead of reporting "No type declaration found". Closes #77.

Both follow the standard exception-stub pattern (extends Exception, four universal constructors, bodies throwing UnsupportedOperationException), matching MathException.

Salesforce Documentation

These exceptions are real but oddly undocumented, they appear only in prose and only in the original preview note:

• EA note (248 / Spring '24): build() triggers FormulaValidationException, evaluate() triggers FormulaEvaluationException
• GA note (254): announces GA, drops all mention of them
• FormulaEval namespace ref: lists the other classes, not these exceptions
• IdeaExchange origin

NOTE: No // [doc link] comment on either class since no reference page exists to point at.

Testing/Evidence

Run the following Apex snippet against an org on API version 67.0:

// Reference the types explicitly so the compiler must resolve them under System.
System.Type validationType = System.FormulaValidationException.class;
System.Type evaluationType = System.FormulaEvaluationException.class;
System.debug('Resolved type (validation): ' + validationType.getName());
System.debug('Resolved type (evaluation): ' + evaluationType.getName());

// FormulaValidationException — thrown at build() time by an invalid formula.
try {
    Formula.builder()
        .withType(Account.SObjectType)
        .withReturnType(FormulaEval.FormulaReturnType.Decimal)
        .withFormula('NOT_A_FUNCTION(1)')
        .build()
        .evaluate(new Account());
} catch (System.FormulaValidationException fe) {
    System.debug('Caught System.FormulaValidationException: ' + fe.getMessage());
}

// FormulaEvaluationException — thrown at evaluate() time (division by zero).
try {
    Formula.builder()
        .withType(Account.SObjectType)
        .withReturnType(FormulaEval.FormulaReturnType.Decimal)
        .withFormula('1 / 0')
        .build()
        .evaluate(new Account());
} catch (System.FormulaEvaluationException fee) {
    System.debug('Caught System.FormulaEvaluationException: ' + fee.getMessage());
}

Additional Changes

• .github/copilot-instructions.md tweaks: "current calendar year" instead of hardcoded 2025, v64/Summer '25 bumped to v67/Summer '26, native-type allowlist spelled out, full header example
• Ensure that Claude picks up the project instructions by adding a new .claude/CLAUDE.md which just references .github/copilot-instructions.md