Fix flaky strRemovePrefix property test asserting wrong invariant

[Copilot]

Bug Fix

What was the bug?

The property at tests/stats/string_ops.test.ts:356 asserted an invariant that strRemovePrefix does not satisfy. strRemovePrefix mirrors Python's str.removeprefix() and strips at most one leading occurrence, but the test claimed the result must never start with prefix when str did:

return !(result.startsWith(prefix) && str.startsWith(prefix));

Any input where the prefix repeats at the start (e.g. str = "aa", prefix = "a") is a legitimate counterexample. The reported ["",""] counterexample was two control characters (file separator, \x1c) that render as empty in the console — control characters were a red herring; the underlying invariant was simply wrong. Flakiness depended on whether shrinking happened to land on such inputs for a given seed.

How did you fix it?

Replaced the property with the actual contract of str.removeprefix(): when the prefix is present, exactly one leading occurrence is stripped; otherwise the string is returned unchanged.

fc.property(fc.asciiString(), fc.asciiString({ minLength: 1 }), (str, prefix) => {
  const result = strRemovePrefix(str, prefix);
  return str.startsWith(prefix) ? prefix + result === str : result === str;
}),

The implementation in src/stats/string_ops.ts was already correct and is unchanged.

Testing

Ran bun test tests/stats/string_ops.test.ts repeatedly; all 54 tests pass deterministically.