refactor: fix all 105 PHPStan 2 strict type findings across core and modules

Resolve all non-false-positive PHPStan 2 findings, reducing the baseline
from 486 to 382 entries (only unused-public false positives remain).

Categories fixed:
- Remove 39 redundant type checks (is_string, is_array, is_object, method_exists)
- Remove 12 no-op array_values() on already-sequential lists
- Simplify 13 null-coalesce on guaranteed offsets
- Remove 8 always-true instanceof checks
- Replace 12 redundant test assertions (assertTrue(true) → addToAssertionCount)
- Simplify 6 unnecessary nullsafe operators (?-> → ->)
- Fix 6 always-true !== '' comparisons
- Fix remaining: unset.offset, return.unusedType, staticMethod narrowing

53 files changed across core/, modules/, pages/, and tests/.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/App/Module/ModuleRuntimeAssetPublisher.php

@@ -34,9 +34,6 @@ final class ModuleRuntimeAssetPublisher
 
         $desired = [];
         foreach ($modules as $manifest) {
-            if (!$manifest instanceof ModuleManifest) {
-                continue;
-            }
             $moduleWebDir = rtrim($manifest->basePath, '/') . '/web';
             if (!is_dir($moduleWebDir)) {
                 continue;