Make plvt_inject_animation_duration() reusable for all transition types, including default fade

[mukeshpanchal27]

Summary

Follow-up to #2040

In #2051, we introduced the plvt_inject_animation_duration() function to support custom animation durations for non-fade transitions. For the default fade transition, however, we had to apply animation-duration directly since it doesn’t support the CSS variable --plvt-view-transition-animation-duration.

This PR refactors and improves the implementation by making plvt_inject_animation_duration() reusable across all transition types including the default fade effect. It ensures consistent handling of animation durations, whether via CSS variables or direct CSS properties, depending on what the browser supports for a given transition type.

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.14%. Comparing base (ac22ba5) to head (d7a45c3).
Report is 4 commits behind head on trunk.

Files with missing lines	Patch %	Lines
plugins/view-transitions/includes/theme.php	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk    #2078      +/-   ##
==========================================
- Coverage   67.16%   67.14%   -0.03%     
==========================================
  Files          93       93              
  Lines        7745     7740       -5     
==========================================
- Hits         5202     5197       -5     
  Misses       2543     2543              

Flag	Coverage Δ
multisite	67.14% <33.33%> (-0.03%)	⬇️
single	36.53% <0.00%> (+0.02%)	⬆️

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[github-actions]

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Co-authored-by: mukeshpanchal27 <mukesh27@git.wordpress.org>
Co-authored-by: felixarntz <flixos90@git.wordpress.org>

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[felixarntz]

@mukeshpanchal27 Neat, I love this - great catch!