Fix Overlapping TTS Playback when different characters are speaking in turns

[JoeyBelvar]

Intent: harden media playback cleanup so interrupted NPC audio is stopped reliably and disposal races do not throw.

• Guards DonePlayingCheck() against player/stream disposal races.
• Centralizes MediaObject stop/dispose cleanup so playback handles are captured before fields are cleared.
• Stops existing NPC media before replacing its dictionary entry, preserving the only handle able to halt stale playback.
• Adds focused NPC audio stop diagnostics for found/missing/failed stop paths.
• Uses TryGetValue() in NPC media stop/replacement paths to avoid dictionary lookup races.

Supports: Sebane1/RoleplayingVoiceDalamud#61