feat: Add handlebars helpers for math, arrays, and json

[tbuckley]

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Bug: Handlebars Helper Bugs: Type Errors and Name Collisions

The recent addition of Handlebars helpers introduces two issues:

1. Array Helper Flaws: The first, last, rest, length, and slice array helpers lack runtime type checking, causing errors when used with non-array inputs (e.g., null, undefined, strings). Additionally, their TypeScript types are incorrectly string[] instead of any[], leading to compilation errors for non-string arrays.
2. Name Collisions: Many new helpers (first, last, rest, length, slice, add, sub, mult, div, json) use common names. Due to Handlebars' helper precedence, these helpers shadow existing variables with the same names, causing breaking changes in templates that previously accessed those variables (e.g., {{ first }} now invokes the helper).

src/lib/utils/HandlebarsPromptFormatter.ts#L34-L43

evals-pwa/src/lib/utils/HandlebarsPromptFormatter.ts

Lines 34 to 43 in bd4c28d

	// Array
	Handlebars.registerHelper('first', (a: string[]) => a[0]);
	Handlebars.registerHelper('last', (a: string[]) => a[a.length - 1]);
	Handlebars.registerHelper('rest', (a: string[]) => a.slice(1));
	Handlebars.registerHelper('length', (a: string[]) => a.length);
	Handlebars.registerHelper('slice', (a: string[], start: number, end: number) =>
	a.slice(start, end),
	);
	// JSON
	Handlebars.registerHelper('json', (a: unknown) => JSON.stringify(a));

Fix in Cursor • Fix in Web

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