reject over-long read counts in Mp4parseIo read impl

[Chandan3456]

the Read impl for Mp4parseIo already refuses a buf longer than isize::MAX, but it hands the callback's return value straight back as Ok(rv as usize) once it's non-negative. a callback that reports more bytes than the buf.len() it was given breaks the Read contract that the count never exceeds the buffer, and the count then flows into Take/read_exact/read_to_end which trust it to stay in bounds (read_exact slices buf[n..], read_to_end grows by n). i noticed it reading the FFI shim right after the isize guard, where the second half of the same check was missing. this errors out when rv as usize > buf.len(), matching the existing guard, and adds a test with a callback that over-reports so the invalid count surfaces as an error instead of Ok.