Fix: add validation to tables for relational queries

[LucaMarconato]

• Add validation for the table returned by relational queries APIs
• Refactor relational queries #1131 (unreleased), accidentally changed the return type for region metadata (spatialdata_attrs). Before the Refactor relational queries #1131, for region of cardinality 1, it was a str, with Refactor relational queries #1131 it became a list of len 1, now it restores str.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.44%. Comparing base (471ae71) to head (0b9015b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1160   +/-   ##
=======================================
  Coverage   92.43%   92.44%           
=======================================
  Files          51       51           
  Lines        7815     7820    +5     
=======================================
+ Hits         7224     7229    +5     
  Misses        591      591           

Files with missing lines	Coverage Δ
src/spatialdata/_core/query/relational_query.py	95.32% <100.00%> (+0.05%)	⬆️

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