FSST shared dictionary encoding results in `DictionaryType::Single`

[CommanderStorm]

Bug Description

Currently, when a FSST shared dictionary is encoded, this results in DictionaryType::Single. This is very likely a bug, since that does not make a lot of sense.

Expected Behavior

Encoding a FSST shared dictionary should not produce DictionaryType::Single. It should be reviewed to ensure the correct dictionary type is used.

Steps to Reproduce

• Encode a FSST shared dictionary with the current implementation.
• Observe that the result is DictionaryType::Single.

Additional Context

This behavior appears inconsistent with the intended usage of shared dictionaries.