Add missing test case for isVisible when media_details is undefined

[Copilot]

Addresses feedback from #74015 to add missing test coverage in mediaDimensionsField.isVisible for when media_details is entirely undefined.

Changes

• Added test case 'returns false when media_details is missing' to the isVisible describe block
• Matches existing test pattern in getValue tests (lines 116-124)
• Validates optional chaining behavior: item?.media_details?.width && item?.media_details?.height

The implementation already handles this case correctly via optional chaining; this adds explicit test coverage for it.

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[Copilot]

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