Missing and near-size image styles 404 because the Drimage subscriber runs after the router

Problem/Motivation

DrimageSubscriber listens on kernel.request at priority 0, after core's RouterListener (32), so a request for a style that does not exist yet 404s in routing first.

• New sizes fail on the first request. The subscriber only runs inside the 404 sub-request, creates the style there and returns the image inside the 404 response (image/webp, and on some hosts a malformed HTTP/2 response a CDN reports as 520). The second request succeeds.
• Near sizes never succeed. Within ratio_distortion of an existing style, findImageStyle() reuses that style and never creates the requested name, so the URL 404s every time (e.g. drimage_improved_focal_720_414 while _720_406 exists). One site had 67 image URLs failing this way.

Seen on 1.0.13, Drupal 11.4.7, Varbase 11 with focal_point, single-site and multisite. Related: #3467535: Fix hitting the page with images for the first time results in having images with `page not found` warnings in the syslog.

Steps to reproduce

1. Install with focal_point and use the Drimage formatter.
2. Request /sites/default/files/styles/drimage_improved_focal_533_300/public/IMAGE.webp for an existing image and a size with no style yet: 404 first, 200 after.
3. Request a size one pixel from an existing style: 404 every time.

Proposed resolution

Subscribe at priority 33 in getSubscribedEvents(), before the router. The first request then returns 200 for missing and near sizes alike. Deployed on a live Varbase 11 multisite, all 107 image URLs that had failed now return 200.

Remaining tasks

• ✅ File an issue
• ✅ Addition/Change/Update/Fix
• ❌ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
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• ❌ Code review by maintainers
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• ❌ Credit contributors
• ❌ Review with the product owner
• ✅ Update Release Notes
• ✅ Release

User interface changes

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API changes

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Data model changes

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Release notes snippet

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