fix: strip language prefix from full_slug when Translatable Slugs app is not installed

[silasjoisten]

Problem

When a story is fetched with the ?language= parameter and the Translatable Slugs app is not installed in the space, the Content Delivery API returns:

• default_full_slug: null
• full_slug: prefixed with the requested language, e.g. en/about-us

ResolveControllerListener uses $story['default_full_slug'] ?? $story['full_slug'] and feeds that slug back into StoriesApi::bySlug() for its second fetch (when the controller declares resolveRelations/resolveLinks) and into the controller registry slug lookup. The API does not know the language-prefixed slug and responds with 404 — so every non-default-locale page whose controller declares resolveRelations or resolveLinks throws a StoryNotFoundException, while the default locale keeps working (its full_slug is never prefixed).

With ascending_redirect_fallback enabled the symptom is even more confusing: the fallback then requests the parent slug (usually a folder) and surfaces its 404, e.g. HTTP/2 404 returned for "https://api.storyblok.com/v2/cdn/stories/raeume?language=en".

Fix

When default_full_slug is null and the story's lang is not default, strip the {lang}/ prefix from full_slug before the slug is reused. Spaces with the Translatable Slugs app installed are unaffected (default_full_slug takes precedence as before).

Testing

• New regression test: story with default_full_slug: null, full_slug: "en/…", lang: "en" and a controller declaring resolveLinks — asserts both bySlug() calls receive the unprefixed slug and the controller still resolves by slug.
• Full suite green (512 tests), PHPStan clean (one baseline count adjusted for the new test).
• Verified end-to-end against a real space without Translatable Slugs: all non-default-locale pages 404 before, 200 after.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.32%. Comparing base (eb0cb4f) to head (249f9c6).

Additional details and impacted files

@@             Coverage Diff              @@
##                1.x     #119      +/-   ##
============================================
+ Coverage     94.30%   94.32%   +0.01%     
- Complexity      476      480       +4     
============================================
  Files            54       54              
  Lines          1493     1498       +5     
============================================
+ Hits           1408     1413       +5     
  Misses           85       85              

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