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/helpdesk/lib/Module/Helpdesk/Service/DebitorDetailService.php

@@ -645,7 +645,7 @@ class DebitorDetailService
 
             $billToNo = trim((string) ($contract['Bill_to_Cust_No'] ?? ''));
             $sellToNo = trim((string) ($contract['Sell_to_Cust_No'] ?? ''));
-            if ($customerNo !== '' && $billToNo !== $customerNo && $sellToNo !== $customerNo) {
+            if ($billToNo !== $customerNo && $sellToNo !== $customerNo) {
                 continue;
             }
 
@@ -1178,7 +1178,7 @@ class DebitorDetailService
         $days = isset($matches[1]) && $matches[1] !== '' ? (int) $matches[1] : 0;
         $hours = isset($matches[2]) && $matches[2] !== '' ? (int) $matches[2] : 0;
         $minutes = isset($matches[3]) && $matches[3] !== '' ? (int) $matches[3] : 0;
-        $secondsFloat = isset($matches[4]) && $matches[4] !== '' ? (float) $matches[4] : 0.0;
+        $secondsFloat = isset($matches[4]) ? (float) $matches[4] : 0.0;
         $seconds = (int) floor($secondsFloat);
 
         return ($days * 86400) + ($hours * 3600) + ($minutes * 60) + $seconds;