Scrub invalid UTF-8 characters in decompress testoutput migration (#2971)

We encountered the following error while attempting to run this
migration for slicer.cdash.org

```
SQLSTATE[22021]: Character not in repertoire: 7
ERROR:  invalid byte sequence for encoding "UTF8": 0xe2 0x43 0x6f
```

Author: zackgalbreath

database/migrations/2025_06_16_152800_decompress_test_output.php

@@ -38,7 +38,15 @@ public function up(): void
                 if ($output === false) {
                     throw new Exception('Error reading stream from database.');
                 }
-                $values[] = $this->decompressOutput($output);
+                $decompressed = $this->decompressOutput($output);
+                if (mb_detect_encoding($decompressed, 'UTF-8', true) === false) {
+                    $decompressed = mb_convert_encoding($decompressed, 'UTF-8', 'UTF-8');
+                    if ($decompressed === false) {
+                        echo "Unable to convert testoutput #{$row->id} to UTF-8\n";
+                        $decompressed = '';
+                    }
+                }
+                $values[] = $decompressed;
             }
 
             // Remove the trailing comma...