Issue #3533113 Drop webp check.

Closes #3533113

What the change does

Drupal.drimage_improved.webp was written by the probe and never read: the format comes from the formatter settings (core_webp / imageapi_optimize_webp) and the browser picks the WebP <source> by itself. The probe only cost every page an extra data URI decode, which is what @andreasderijcke reported. Removed from both scripts, the module's and Drimage S3fs's.

WebP is now also only requested when it can actually be delivered: the formatter checks that the image toolkit lists webp among its supported extensions, and that imageapi_optimize_webp is installed, instead of trusting the stored setting alone. A site that loses that support stops emitting WebP URLs.

Testing

• Kernel: DrimageFormatterTest::testWebpOnlyWhenSupported asserts the data follows real support and that no type="image/webp" source is rendered when it is off. Full suite 33 tests / 192 assertions, phpcs and cspell clean.
• Real browser on Varbase 10.1.x (Drupal 11.4.5) and on plain Drupal 11.4.5: checkWebp is gone, 6/6 images decode, 6 WebP sources present, 8 .webp derivatives served, 0 data URI probe requests.

Thanks @andreasderijcke for spotting it.

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