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>

tests/Architecture/ModuleUiContractTest.php

@@ -95,7 +95,7 @@ final class ModuleUiContractTest extends AbstractModuleStructureContractTestCase
                     "Module '{$module['id']}' scheduler_jobs[{$index}] default_schedule_type invalid"
                 );
 
-                $allowedTypes = array_values($job['allowed_schedule_types']);
+                $allowedTypes = $job['allowed_schedule_types'];
                 self::assertNotEmpty($allowedTypes, "Module '{$module['id']}' scheduler_jobs[{$index}] allowed_schedule_types must be non-empty");
                 foreach ($allowedTypes as $allowedType) {
                     self::assertContains(