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/Service/Scheduler/SchedulerRunService.php

@@ -196,7 +196,7 @@ class SchedulerRunService
                 $errorCode = 'job_not_registered';
             } else {
                 $execution = $this->scheduledJobRegistry->execute((string) $job['job_key'], $actorUserId);
-                $status = ScheduledJobRunStatus::tryNormalize((string) $execution['status'])?->value
+                $status = ScheduledJobRunStatus::tryNormalize((string) $execution['status'])->value
                     ?? ScheduledJobRunStatus::Failed->value;
                 $errorCode = $this->nullableString($execution['error_code']);
                 $errorMessage = $this->nullableString($execution['error_message']);