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>

modules/audit/lib/Module/Audit/Service/FrontendTelemetryIngestService.php

@@ -485,11 +485,7 @@ class FrontendTelemetryIngestService
 
         $cutoff = $now - self::DEDUPE_WINDOW_SECONDS;
         foreach ($seen as $key => $timestamp) {
-            if (!is_string($key)) {
-                unset($seen[$key]);
-                continue;
-            }
-            if (!is_int($timestamp) || $timestamp < $cutoff) {
+            if ($timestamp < $cutoff) {
                 unset($seen[$key]);
             }
         }