fix(tui): correct truncateLeft output when len is 1

[chinesepowered]

Issue for this PR

Closes #37368

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

truncateLeft(str, len) ended with "…" + str.slice(-(len - 1)). When len === 1 that is str.slice(-0), and slice(-0) equals slice(0) — the whole string — so the function returned something longer than the input (truncateLeft("abcdef", 1) → "…abcdef").

This replaces the negative index with an equivalent positive one: str.slice(str.length - (len - 1)). At len === 1 that becomes str.slice(str.length) → "", giving "…". For every len >= 2 the two expressions are identical, so existing behavior is unchanged. This also brings it in line with the sibling truncate(), which already handles len === 1 correctly.

How did you verify your code works?

Ran the old and new implementations side by side over a range of lengths:

• len === 1: old "…abcdef" (7 chars) → new "…" (1 char) ✅
• len === 2, 3, 5: identical output to before ("…f", "…ef", "…cdef") ✅
• len >= str.length: returns the string unchanged ✅

One-character diff, no caller changes needed.

Screenshots / recordings

N/A — not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR