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/Import/CsvReaderService.php

@@ -126,9 +126,6 @@ class CsvReaderService
         $lineNumber = 0;
         while (($row = fgetcsv($handle, 0, $delimiter, '"', '\\')) !== false) {
             $lineNumber++;
-            if (!is_array($row)) {
-                continue;
-            }
             $row = $this->normalizeRowColumns($row, count($row));
             if ($this->isEmptyRow($row)) {
                 continue;
@@ -207,9 +204,6 @@ class CsvReaderService
             if ($lineNumber <= $headerLine) {
                 continue;
             }
-            if (!is_array($row)) {
-                continue;
-            }
             $row = $this->normalizeRowColumns($row, count($headers));
             if ($this->isEmptyRow($row)) {
                 continue;