Fix PHPStan type safety issues in DimensionsRecordTest

Add proper type assertions to handle potential false returns:
- Assert file_get_contents() returns string, not false
- Assert strpos() returns int, not false
- Assert unpack() returns array, not false

This resolves all 7 PHPStan errors reported in CI while maintaining
test functionality (12 assertions, all passing).

Author: zlatanovic-nebojsa

tests/PhpSpreadsheetTests/Writer/Xls/DimensionsRecordTest.php

@@ -45,20 +45,22 @@ public function testDimensionsRecordUsesZeroBasedColumnIndices(): void
 
         // Read the binary file and find the DIMENSIONS record
         $fileContent = file_get_contents($filename);
+        self::assertIsString($fileContent, 'Failed to read XLS file');
         unlink($filename);
 
         // Find the DIMENSIONS record: 0x0200 (2 bytes) + length 0x000E (2 bytes)
         $recordSignature = pack('v', 0x0200) . pack('v', 0x000E);
         $pos = strpos($fileContent, $recordSignature);
 
-        self::assertNotFalse($pos, 'DIMENSIONS record not found in XLS file');
+        self::assertIsInt($pos, 'DIMENSIONS record not found in XLS file');
 
         // Parse the DIMENSIONS record (skip 4-byte header)
         $dataPos = $pos + 4;
         $dimensionsData = substr($fileContent, $dataPos, 14);
 
         // Unpack DIMENSIONS record
         $data = unpack('VrwMic/VrwMac/vcolMic/vcolMac/vreserved', $dimensionsData);
+        self::assertIsArray($data, 'Failed to unpack DIMENSIONS record');
 
         // Verify the values are correct (0-based for columns, 1-based for rows)
         // First used row is 1
@@ -100,20 +102,22 @@ public function testDimensionsRecordCapsColumnIndexAt255(): void
 
         // Read the binary file and find the DIMENSIONS record
         $fileContent = file_get_contents($filename);
+        self::assertIsString($fileContent, 'Failed to read XLS file');
         unlink($filename);
 
         // Find the DIMENSIONS record: 0x0200 (2 bytes) + length 0x000E (2 bytes)
         $recordSignature = pack('v', 0x0200) . pack('v', 0x000E);
         $pos = strpos($fileContent, $recordSignature);
 
-        self::assertNotFalse($pos, 'DIMENSIONS record not found in XLS file');
+        self::assertIsInt($pos, 'DIMENSIONS record not found in XLS file');
 
         // Parse the DIMENSIONS record (skip 4-byte header)
         $dataPos = $pos + 4;
         $dimensionsData = substr($fileContent, $dataPos, 14);
 
         // Unpack DIMENSIONS record
         $data = unpack('VrwMic/VrwMac/vcolMic/vcolMac/vreserved', $dimensionsData);
+        self::assertIsArray($data, 'Failed to unpack DIMENSIONS record');
 
         // Last column should be capped at 256 (255 + 1 for "last used + 1")
         // The min(255, ...) ensures we don't exceed the BIFF8 limit