[AI assisted] MM-62927: add comprehensive whitespace validation tests for FileSettings paths (#31087)

https://mattermost.atlassian.net/browse/MM-62927

```release-note
NONE
```
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/i18n/en.json

@@ -9152,6 +9152,10 @@
     "id": "model.config.is_valid.directory.app_error",
     "translation": "Invalid Local Storage Directory. Must be a non-empty string."
   },
+  {
+    "id": "model.config.is_valid.directory_whitespace.app_error",
+    "translation": "Leading or trailing whitespace detected for {{.Setting}}. Found \"{{.Value}}\"."
+  },
   {
     "id": "model.config.is_valid.display.custom_url_schemes.app_error",
     "translation": "The custom URL scheme {{.Scheme}} is invalid. Custom URL schemes must start with a letter and contain only letters, numbers, plus (+), period (.) and hyphen (-)."