Fix parsing/serialization of packed proto3 custom options

We have an obscure bug where packed-by-default repeated custom options defined
in proto3 files are not correctly treated as packed when dynamic extensions are
used. This CL introduces a simple fix and adds a test verifying the correct
behavior.

The bug was not a major problem in most cases, since parsers will accept both
the packed and unpacked encodings. It is useful to fix this to ensure that
descriptors are serialized in a consistent way, though.

PiperOrigin-RevId: 513249881