fix: crash on completion in sql mode

Summary

• fixed crash on completion in sql mode for picodata admin
• serialization of sql output in sql mode for picodata admin

Bug:

1. connect to active instance via picodata admin
2. change language to sql via \sql
3. enter tab twice (probably after some text)
4. console crashes after try to complete input in sql mode

Fix: Turn off completion for sql mode in picodata admin

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• Target branch: master
• Docs follow-up: not necessary
• Follow-up for !833 (merged)