fix: avoid "No clang found" when the clang cache is deleted by a parallel request

[gharlan]

Fixes #6648

Problem

rex_clang::checkCache() used a check-then-read pattern:

if (!is_file($file)) {
    rex_clang_service::generateCache();
}
foreach (rex_file::getCache($file) as $id => $data) {

rex_delete_cache() wipes the whole cache directory. If that happens between the is_file() check and the read, file_get_contents() fails, rex_file::getCache() returns its empty default [] — and checkCache() still sets cacheLoaded = true. The request then runs with zero languages.

The visible symptom is LogicException: No clang found. from getStartId(), which boot.php evaluates on nearly every request (rex_request('clang', 'int', rex_clang::getStartId())) — hence the reports of the error hitting completely unrelated pages. Less visible, but worse: exists(), get() and getAll() silently return wrong results in the same situation.

Note that the "reading while another request writes" theory from the issue does not apply — rex_file::put() already writes atomically via tempnam() + rename().

Fix

Read the file first and fall back to a freshly generated cache when the result is empty. rex_clang_service::generateCache() now returns the data it generated, so the fallback does not read the file back from disk and cannot lose the same race a second time. This mirrors the read-first pattern already used by mediapool and structure.

As a side effect, an empty or truncated cache file no longer leads to foreach over null.

The exception in getStartId() still fires when the clang table really is empty.

Compatibility

generateCache() gained a return value (previously @return void); callers are unaffected.

Testing

• Manually verified with a deleted and with a truncated clang.cache: the cache is regenerated and the languages load correctly.
• composer phpstan, composer psalm, composer cs clean; no baseline changes.

[gharlan]

@skerbis Da nicht so direkt reproduzierbar bei dir, ist es wahrscheinlich nicht so leicht testbar für dich, oder? Dann würde ich es einfach so mergen und hoffen, dass es tatsächlich hilft.

[skerbis]

@gharlan danke. ja es ist schwer zu testen.