strings: fix `lpad`/`rpad` remainder with a `Char` pad (#62837)
When the pad width does not evenly divide the remaining columns,
`lpad`/`rpad` filled the last `r` columns with `first(p, r)`. For a
`Char` pad `p` this iterates the char into a `Vector{Char}`, which is
then stringified into the result:
```julia-repl
julia> rpad("x", 6, '🍕')
"x🍕🍕['🍕']"
```
Use the whole char instead, matching the string-pad behavior of
overshooting the requested width. This also fixes `lpad`/`rpad` failing
`--trim=safe` verification through this code path.
