Fix panic in format() on negative replacement index

[semx]

Description

format() panics with runtime error: index out of range [-1] when a placeholder uses a negative index, e.g. format('{-1}', 'x').

When parsing a {N} placeholder, the - and digits are collected into replacementIndex, so strconv.ParseInt("-1", 10, 32) succeeds and returns -1. The only guards were a parse error and an index that is too large (len(replaceValue) <= int(index)), which is false for a negative index. Execution then reaches replaceValue[index] with a negative index and the runtime panics.

GitHub Actions instead returns a handled expression error for such input, the same way act already handles the out-of-range case {2147483648}.

Reproduction

NewInterpeter(&EvaluationEnvironment{}, Config{}).Evaluate("format('{-1}', 'x')", DefaultStatusCheckNone)

Before: panic: runtime error: index out of range [-1]
After: returns error The following format string is invalid: '{-1}'

Fix

Reject a negative index alongside the existing parse-error guard. A test case format('{-1}', 'x') is added next to the existing {2147483648} case.

[louzt]

Verified locally against act master HEAD 4f41128 — the index < 0 bounds check correctly handles format('{-1}', 'x'), returning the same controlled error as the existing {2147483648} overflow case. The new format-invalid-negative-index test case is a clean regression test. Happy to retest if you rebase or want a second reviewer.