Media library selection only populates sibling image attributes for *.image.src keys

Problem/Motivation

Selecting an image from the media library in URL mode (media_token=0) currently populates sibling image attributes (for example alt, title, width, height) only when the target widget path ends with .image.src. Schemas using other valid object keys such as custom_image.src or alternate_media.src receive the src value but miss sibling attribute population.

Steps to reproduce

1. Create or use a Patternkit schema with a non-image object key that still follows a *.src shape, for example custom_image.src, custom_image.alt, custom_image.width, custom_image.height, and custom_image.title.
2. Open Layout Builder and add the block using that schema.
3. Click Select/Upload image for the non-image key's src field.
4. Choose an image and click Insert selected.
5. Observe the resulting field values.

Note: This patch adds the [Patternkit] Custom key image attributes test pattern in patternkit_test; when patternkit_test is enabled, use that pattern to verify the fix directly instead of reproducing from a separate feature-branch-only schema.

Proposed resolution

Update opener-side gating to use a generic *.src suffix check instead of hard-coding .image.src, then cover selection-driven behavior with a regression test that validates multiple non-image object keys.

Code changes

• modules/patternkit_media_library/src/MediaLibraryJSONLibraryOpener.php: change sibling-population gate from .image.src to .src while preserving silent-skip behavior when sibling fields are absent.

Test coverage

• modules/patternkit_media_library/tests/src/FunctionalJavascript/JsonEditorMediaSelectionTest.php: add testImageAttributesPopulateOnSelectionForNonImageObjectKeys() to verify selection-driven attribute population for custom_image and alternate_media key paths.
• tests/modules/patternkit_test/lib/patternkit_test/components/custom_key_image_attributes_test/custom_key_image_attributes_test.json and tests/modules/patternkit_test/lib/patternkit_test/components/custom_key_image_attributes_test/custom_key_image_attributes_test.twig: add a reusable test pattern with two non-image object keys for verification.
• Retain existing blur-driven coverage (testImageAttributesFetchOnBlur()) as adjacent validation while this issue targets selection-flow opener behavior.

Related issues

• Historical context: #3370254: Adding an image url via Media Library should also populate image alt, height, and width text fields.

Remaining tasks

• Review and test the merge request.
• CI validation.
• Confirm FunctionalJavascript coverage passes in local and CI environments.

User interface changes

None.

Introduced terminology

None.

API changes

None.

Data model changes

None.

Release notes snippet

Media-library image selection now populates sibling image attributes for any schema path ending in *.src, not only paths ending in *.image.src, improving compatibility with custom schema object key names.