Fix critical bug in truncateStringWithMessage: Prevent negative slice indices - #1209

Good catch on the underlying issue — str.slice(0, maxLength - suffix.length) with a negative argument does count from the end rather than truncating, so for small maxLength values the function silently returns most of the original string plus the truncation banner. The Math.max(0, ...) guards for END and START modes in common/src/util/string.ts fix that correctly, and common/ is in scope per the contributing guide.

However, the MIDDLE-mode fix doesn't actually solve the problem it claims to. When maxLength is small enough that length clamps to 0, str.slice(-length) becomes str.slice(-0), and -0 === 0 in JS, so slice(-0) is identical to slice(0) — it returns the entire string, not an empty one. So for small maxLength in MIDDLE mode you still get the whole original string plus the truncation banner stuffed in the middle, which is exactly the bug this PR is meant to fix.

The added test 'should handle negative available length for MIDDLE mode' doesn't catch this because it only asserts result.toContain('TRUNCATED DUE TO LENGTH'), which is true regardless of whether the string was actually truncated. Please add an assertion checking the total output length (or that the head/tail portions are actually empty) to catch this, and fix the MIDDLE branch, e.g. by using length > 0 ? str.slice(-length) : ''.

Right instinct, solid fix for two of three branches, but the MIDDLE-mode case — arguably the trickiest of the three — is still broken, and the test suite as written wouldn't have caught it.