Rollup merge of #130408 - okaneco:into_lossy_refactor, r=Noratrieb

Avoid re-validating UTF-8 in `FromUtf8Error::into_utf8_lossy`

Part of the unstable feature `string_from_utf8_lossy_owned` - #129436

Refactor `FromUtf8Error::into_utf8_lossy` to copy valid UTF-8 bytes into the buffer, avoiding double validation of bytes.
Add tests that mirror the `String::from_utf8_lossy` tests.

Author: compiler-errors