Uncaught ServiceUnavailableHttpException ("Image generation in progress") when a derivative is being generated concurrently

Problem/Motivation

DrimageSubscriber::onKernelRequest() calls DrimageManager::image(), which extends core ImageStyleDownloadController and calls deliver(). When another request holds the lock image_style_deliver:<style>:<hash>, core throws ServiceUnavailableHttpException. The subscriber does not catch it, so it is logged as an exception and <img> tags never retry: visitors see broken images. Seen on a production site under concurrent requests on cold derivatives.

Symfony\Component\HttpKernel\Exception\ServiceUnavailableHttpException: Image generation in progress. Try again shortly.

Related: #3546893 (same message, closed as works as designed, no code change), #3587334 (return handling in the subscriber), #3626466 (subscriber priority).

Steps to reproduce

1. Install Drupal with Dynamic Responsive Image (Drimage) - Improved 1.0.x-dev, PHP and database as per your environment.
2. Clear the image style derivatives for a Drimage-rendered image.
3. Request the same cold derivative from concurrent clients, or hold the image_style_deliver:<style>:<hash> lock from another process while requesting it.
4. Requests that hit the lock throw the uncaught exception above.

Proposed resolution

In DrimageSubscriber::onKernelRequest(), catch ServiceUnavailableHttpException, retry up to 5 times with usleep(500000), and if still locked answer with a plain Response (503, Retry-After: 3, Cache-Control: no-store) instead of an uncaught exception.

A Kernel test, DrimageSubscriberLockTest, covers both paths: lock held for every attempt (plain 503 with the headers) and lock released during the wait (retried to 200). Unpatched, both tests fail with ServiceUnavailableHttpException.

Also checked with real requests on a DDEV site:

• Lock held about 1.2s: 200 after about 1.6s.
• Lock held 8s: plain 503 with Retry-After: 3 and Cache-Control: no-store after about 2s.
• Missing source file: 404, unchanged. Normal request: 200, unchanged.

Remaining tasks

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API changes

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Data model changes

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