[two_dimensional_scrollables] Add regression test

[Piinks]

Fixes flutter/flutter#137112

This PR adds a regression test for flutter/flutter#137112, where TableView would crash when focusing a widget outside of the table while a previously focused TextField inside the table was scrolled out of view.

While this issue appears to have been resolved by changes in the Flutter framework specifically flutter/flutter#135182 which addressed how focus keeps children alive in two-dimensional scrollables, this PR adds a dedicated test case to the package to ensure this behavior does not regress.

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[Piinks]

Reason for override: Just adding a test. :)

[gemini-code-assist]

Code Review

This pull request adds a regression test to table_test.dart to ensure that TableView does not crash when focus shifts to an external widget while a previously focused cell is scrolled out of view. The reviewer suggests replacing magic numbers used for scroll offsets and cell extents with named constants or calculated values to improve test maintainability and readability.

[AbdeMohlbi]

SGTM