[BUGFIX] `argilla server`: Prevent passing non-string values to text fields

[frascuchon]

Description

This PR prevents creating records with non-string values for text fields. Currently, users can create records with dictionary values as values for text fields.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested

Checklist

• I added relevant documentation
• I followed the style guidelines of this project
• I did a self-review of my code
• I made corresponding changes to the documentation
• I confirm My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.56%. Comparing base (f47f20d) to head (19ff998). Report is 1 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #5682 +/- ## =========================================== + Coverage 91.55% 91.56% +0.01% =========================================== Files 165 165 Lines 6675 6686 +11 =========================================== + Hits 6111 6122 +11 Misses 564 564 ``` | [Flag](https://app.codecov.io/gh/argilla-io/argilla/pull/5682/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=argilla-io) | Coverage Δ | | |---|---|---| | [argilla-server](https://app.codecov.io/gh/argilla-io/argilla/pull/5682/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=argilla-io) | `91.56% <100.00%> (+0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=argilla-io#carryforward-flags-in-the-pull-request-comment) to find out more.

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