Add unit tests for Expression, JsonExpression and TableSchema

[WarLikeLaux]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌
Fixed issues

What does this PR do?

Adds dedicated unit tests for Expression, JsonExpression and TableSchema - pure logic, no DB required.

Before

Tests: 0, Assertions: 0
Methods: 0% (0/9), Lines: 0% (0/24)
MSI: 0% (0/24 mutants killed)

After

Tests: 46, Assertions: 68
Methods: 100% (9/9), Lines: 100% (24/24)
MSI: 100% (24/24 mutants killed)

---

Changelog not required, because no source files changed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.13%. Comparing base (37b6e7b) to head (f2accdc).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #20770   +/-   ##
=========================================
  Coverage     80.13%   80.13%           
  Complexity    11533    11533           
=========================================
  Files           374      374           
  Lines         30206    30206           
=========================================
  Hits          24207    24207           
  Misses         5999     5999           

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