Fix CI failure: bump phpdocumentor/shim minimum version to ^3.5

The shim v3.0.0 (the minimum allowed by ^3) tries to download
phpDocumentor from a GitHub release URL that no longer exists,
causing "lowest" dependency CI jobs to fail with 404 errors.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: jonathanhefner

composer.json

@@ -37,7 +37,7 @@
     "nyholm/psr7": "^1.8",
     "nyholm/psr7-server": "^1.1",
     "php-cs-fixer/shim": "^3.91",
-    "phpdocumentor/shim": "^3",
+    "phpdocumentor/shim": "^3.5",
     "phpstan/phpstan": "^2.1",
     "phpunit/phpunit": "^10.5",
     "psr/cache": "^3.0",