Tests: Add E2E tests for Import/Export workflows

[priethor]

What

Part of #315.

Adds E2E tests for Import/Export workflows.

Why

To ensure import/export functionality works correctly for all entity types.

How

Category	Tests
JSON Export	Export field group, warning when none selected, export multiple at once
PHP Export	Generate PHP code, include all field settings
JSON Import	Invalid JSON error, no file error, empty array error, update on same key
Round Trip	Field group, post type, taxonomy, options page preservation
Overwrite	Post type, taxonomy, options page update on same key

Testing Instructions

Run the E2E tests:

npm run test:e2e -- tests/e2e/import-export.spec.ts

[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 32.73%. Comparing base (af9ae43) to head (5b5e4e1).
⚠️ Report is 2 commits behind head on trunk.

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #345      +/-   ##
============================================
+ Coverage     31.71%   32.73%   +1.01%     
  Complexity     4410     4410              
============================================
  Files           291      291              
  Lines         36918    36918              
============================================
+ Hits          11710    12086     +376     
+ Misses        25208    24832     -376     

Flag	Coverage Δ
e2e	40.84% <ø> (-2.59%)	⬇️
javascript	15.07% <ø> (ø)
phpunit	28.70% <ø> (ø)

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