Throw string-specific index exception in `ParsableByteArray.readString`

This change throws `StringIndexOutOfBoundsException` instead of the
generic `IndexOutOfBoundsException` to maintain compatibility with
existing callers.

We also enable `ParsableByteArray` limit enforcement in
`ParsableByteArrayTest` to prevent backsliding and add tests for the
limit enforcement.

PiperOrigin-RevId: 938546274