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fix(string): abort on negative repeat count and guard against overflow

`String::repeat` and `StringView::repeat` previously matched
`_..=0 => ""`, silently returning an empty string for any non-positive
`n`. Negative `n` is almost always a caller bug (sign error /
off-by-one), so abort with `"negative repeat count"` instead. An `n` of
`0` still returns the empty string.

Also mirror the multiplicative-overflow guard already present in
`Bytes::repeat` so a wrap-around in `len * n` aborts cleanly with
`"repeat result too large"` instead of silently producing a wrong-sized
size hint.

Two existing tests that asserted the old silent-empty behavior on
negative `n` are converted to `panic` tests.

Refs #3498. Split out from #3505 for easier review.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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