Dominant Color Images: Clean up redundant metadata generation in tests

[wprashed]

Description

Fixes #2524.

In Dominant_Color_ImagesTestsTestCase, create_upload_object() automatically invokes wp_generate_attachment_metadata(), which triggers the dominant_color_metadata filter and attaches dominant color data.

Subsequent calls to wp_maybe_generate_attachment_metadata() were redundant because metadata was already populated.

Changes

• Removed redundant wp_maybe_generate_attachment_metadata() calls in class-testcase.php and test-dominant-color.php.
• Updated test_get_dominant_color_valid() to assert directly on the generated _wp_attachment_metadata, testing the end-to-end flow from upload to metadata storage.

Testing Instructions

1. Run composer test:dominant-color-images or run PHPUnit on the dominant color images test suite.
2. Confirm all test assertions pass without regression.

[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.35%. Comparing base (cedbbac) to head (1448132).

Additional details and impacted files

@@           Coverage Diff           @@
##            trunk    #2649   +/-   ##
=======================================
  Coverage   70.35%   70.35%           
=======================================
  Files          91       91           
  Lines        7867     7867           
=======================================
  Hits         5535     5535           
  Misses       2332     2332           

Flag	Coverage Δ
multisite	70.35% <ø> (ø)
single	35.17% <ø> (ø)

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[nickchomey]

I'm curious why this was opened when #2527 already exists to address #2524

[westonruter]

@wprashed Please review #2527 instead of opening your own PR.