Fix extras naming and version assignment - #17456

Changes

• Extras are now named after what they are: "Trailer", "Behind The Scenes", numbered when a type repeats, instead of inheriting the owner's file name
• TryGetExtraTypeForOwner hands back the rule it matched so an extra carrying a real title of its own ("Recording the audio-behindthescenes") keeps it, and a corrected name is persisted on the next refresh.
• Remote metadata and image providers no longer run for extras, since an extra has no identity of its own
• For an item with several versions, an extra named after a specific version file is assigned to that version, while folder-based extras and extras named after the item as a whole stay with the primary
  • Every version surfaces all of them, and the HasTrailer / HasSpecialFeature / HasThemeSong / HasThemeVideo filters match on any version.
  • Alternate versions and stacked parts are owned items but not extras, so they are no longer counted as such when deciding whether an item's extras changed.

Code assistance
Claude Opus 5 was used to trace the issue and write the tests.

Issues