feat: add strings.repeat builtin

[anakrish]

Adds a new strings.repeat(s, count) builtin that repeats a string count times.

Example:

strings.repeat("abc", 3)  # returns "abcabcabc"

This follows the pattern of other string builtins like strings.reverse and strings.count.

[Copilot]

Pull request overview

Adds a new strings.repeat(s, count) builtin to the Rego string builtin set, aligning with other strings.* helpers in src/builtins/strings.rs.

Changes:

• Register a new builtin name strings.repeat with arity 2.
• Implement repeat builtin logic to concatenate a string count times.

[Copilot]

The repeat loop grows result potentially many times without any call to enforce_limit(). Other builtins that accumulate in loops call enforce_limit() to respect the engine’s memory limits; without it, strings.repeat can more easily trigger OOM/DoS for large count. Add limit enforcement during repetition (and/or before growth) so large repeats fail gracefully under configured limits.

Suggested change

	let mut result = String::new();
	for _ in 0..count {
	let part_len = s.len();
	let mut result = String::new();
	for _ in 0..count {
	let next_len = match result.len().checked_add(part_len) {
	Some(next_len) => next_len,
	None => bail!(params[1].span().error("repeat result exceeds supported size")),
	};
	enforce_limit(next_len)?;

[Copilot]

result is built with String::new() and repeated push_str, which can lead to repeated reallocations for larger counts. Consider reserving capacity up front using a checked length computation (e.g., checked_mul of s.len() and count) or otherwise minimizing reallocations, while ensuring the approach can’t panic on overflow.

Suggested change

	let mut result = String::new();
	let capacity = s
	.len()
	.checked_mul(count)
	.ok_or_else(|| params[1].span().error("repeat count produces a string that is too large"))?;
	let mut result = String::new();
	result
	.try_reserve(capacity)
	.map_err(|_| params[1].span().error("repeat count produces a string that is too large"))?;

[Copilot]

A new builtin (strings.repeat) is being added but there are no corresponding interpreter YAML test cases under tests/interpreter/cases/builtins/strings/ (this directory already contains coverage for other string builtins like concat, contains, etc.). Add tests for basic behavior (e.g., count 0/1/N), Unicode handling, and invalid count cases (non-integer / negative) to lock down semantics across execution paths.

[Copilot]

strings.repeat is now registered but the builtin list in docs/builtins.md has a dedicated Strings table that enumerates supported builtins (e.g., strings.replace_n, strings.reverse). Add strings.repeat there as well so the documented builtin surface matches what the engine exposes.

[Copilot]

args[1].as_f64().unwrap() as usize can panic (unwrap), silently truncates non-integers, and will produce a huge usize for negative counts due to casting. This violates the project’s no-panics requirement and yields incorrect semantics for invalid count values. Parse count via ensure_numeric and validate it as a non-negative integer (e.g., using validate_integer_arg), then convert using an explicit checked/option-based conversion (returning Undefined or a strict-mode error as appropriate).

Suggested change

	fn repeat(span: &Span, params: &[Ref<Expr>], args: &[Value], _strict: bool) -> Result<Value> {
	let name = "strings.repeat";
	ensure_args_count(span, name, params, args, 2)?;
	let s = ensure_string(name, &params[0], &args[0])?;
	let count = args[1].as_f64().unwrap() as usize;
	fn repeat(span: &Span, params: &[Ref<Expr>], args: &[Value], strict: bool) -> Result<Value> {
	let name = "strings.repeat";
	ensure_args_count(span, name, params, args, 2)?;
	let s = ensure_string(name, &params[0], &args[0])?;
	let count = ensure_numeric(name, &params[1], &args[1])?;
	let count = match count.as_i64().and_then(|value| usize::try_from(value).ok()) {
	Some(count) => count,
	None if strict => {
	bail!(params[1]
	.span()
	.error("`strings.repeat` expects a non-negative integer count"))
	}
	None => return Ok(Value::Undefined),
	};