Fix NPC chat showing only name without message text

Two bugs in SMSG_MESSAGECHAT parser for MONSTER_SAY/YELL/EMOTE:

1. Sender name included trailing null byte from server (nameLen includes
   null terminator). The embedded null in std::string caused ImGui to
   truncate the concatenated display string at the NPC name, hiding
   " says: <message>" entirely.

2. Missing NamedGuid receiver name for non-player/non-pet targets. When
   the receiver GUID is a creature, the server writes an additional
   SizedCString (target name) that we weren't reading, shifting all
   subsequent field reads.

Also adds MONSTER_WHISPER, MONSTER_PARTY, RAID_BOSS_EMOTE, RAID_BOSS_WHISPER
chat types with proper parsing and display formatting (says/yells/whispers).

include/game/world_packets.hpp

@@ -605,7 +605,11 @@ enum class ChatType : uint8_t {
     RAID_LEADER = 27,
     RAID_WARNING = 28,
     ACHIEVEMENT = 29,
-    GUILD_ACHIEVEMENT = 30
+    GUILD_ACHIEVEMENT = 30,
+    MONSTER_WHISPER = 42,
+    RAID_BOSS_WHISPER = 43,
+    RAID_BOSS_EMOTE = 44,
+    MONSTER_PARTY = 50
 };
 
 /**