Add unit tests for grid widget classes

[WarLikeLaux]

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

What does this PR do?

Adds unit tests for grid widget classes to reach 100% line coverage across all 8 files. Also fixes formatter fallback tests to respect mocked intl status in CI.

Coverage

File	Tests	Lines	MSI
ActionColumn.php	2 → 6	42/47 (89%) → 47/47 (100%)	-
CheckboxColumn.php	3 → 6	31/34 (91%) → 34/34 (100%)	-
Column.php	0 → 7	8/14 (57%) → 14/14 (100%)	-
DataColumn.php	8 → 19	42/62 (68%) → 62/62 (100%)	-
GridView.php	5 → 21	112/141 (79%) → 141/141 (100%)	-
RadioButtonColumn.php	4 → 5	11/12 (92%) → 12/12 (100%)	-
SerialColumn.php	6 → 6	4/4 (100%) → 4/4 (100%)	-
Total	30 → 92	250/314 (80%) → 314/314 (100%)	91%

Changelog not required, because no source files changed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.83%. Comparing base (068c5a4) to head (612b9e8).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #20788      +/-   ##
============================================
+ Coverage     78.62%   78.83%   +0.21%     
  Complexity    11566    11566              
============================================
  Files           376      376              
  Lines         30748    30748              
============================================
+ Hits          24176    24241      +65     
+ Misses         6572     6507      -65     

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