Fix empty chat messages and /dismount on Classic/Turtle

Chat: renderTextWithLinks now properly handles |cAARRGGBB color codes
that aren't item links (e.g. colored player names), rendering the text
in the specified color instead of discarding it.

Dismount: Classic/Vanilla lacks CMSG_CANCEL_MOUNT_AURA (TBC+ opcode).
Track mount aura spell ID when mountDisplayId changes, then use
CMSG_CANCEL_AURA as fallback on expansions without the dedicated opcode.

include/game/game_handler.hpp

@@ -1401,6 +1401,7 @@ private:
     TaxiOrientationCallback taxiOrientationCallback_;
     TaxiFlightStartCallback taxiFlightStartCallback_;
     uint32_t currentMountDisplayId_ = 0;
+    uint32_t mountAuraSpellId_ = 0;       // Spell ID of the aura that caused mounting (for CMSG_CANCEL_AURA fallback)
     float serverRunSpeed_ = 7.0f;
     bool playerDead_ = false;
     bool releasedSpirit_ = false;