Fix multivendor_master build after upstream master merge

[Rochet2]

Summary

The scheduled merge workflow merges TrinityCore/TrinityCore master into multivendor_master and builds with -DWITH_WARNINGS_AS_ERRORS=1. The branch was failing at the merge step (unresolved conflict in GossipDef.h) and would not compile after upstream refactored player interaction state.

This PR merges the latest upstream master into multivendor_master, resolves the conflict, and adapts the multivendor virtual-vendor tracking to the new upstream InteractionData API.

What was wrong

Merge conflict

1. GossipDef.h — Upstream split the old flat InteractionData struct into a variant-based design (TrainerData, PlayerChoiceData, …) and moved PlayerChoiceData to its own class. The multivendor branch still stored the gossip-selected vendor entry as a flat VendorId field on the old struct; Git left the two incompatible class definitions as a conflict.

Post-merge compile errors

2. ItemHandler.cpp::SendListInventory — Multivendor passes an optional vendorEntry (from gossip ActionMenuID) and stores it for later buy validation. After the merge, code still referenced the removed VendorId field and also incorrectly called StartInteraction() and then immediately Reset() + manual field assignment.

3. Player.cpp::BuyItemFromVendorSlot — Anti-cheat / item-list lookup still read GetInteractionData().VendorId to load the virtual vendor item list when buying from a gossip-selected multivendor window.

4. cs_npc.cpp — .npc add vendoritem with the multivendor flag still read .VendorId; .npc delete vendoritem with the flag incorrectly used .TrainerId instead of the active vendor entry.

What was fixed

File	Fix
src/server/game/Entities/Creature/GossipDef.h	Keep upstream PlayerChoiceData + variant InteractionData; add VendorData (mirroring TrainerData) with GetVendor() accessors.
src/server/game/Entities/Creature/GossipDef.cpp	Emplace VendorData when StartInteraction(..., PlayerInteractionType::Vendor) is called.
src/server/game/Handlers/ItemHandler.cpp	After StartInteraction, store gossip vendorEntry in GetVendor()->Id; keep loading items via GetNpcVendorItemList(vendorEntry) when set.
src/server/game/Entities/Player/Player.cpp	Read the active virtual vendor entry from GetVendor()->Id when validating purchases.
src/server/scripts/Commands/cs_npc.cpp	Use GetVendor()->Id for multivendor .npc add/delete vendoritem; fix delete path that used TrainerId.

Local verification

• Re-ran CMake against the existing BUILD directory.
• Built game.vcxproj and scripts.vcxproj (Debug) successfully with VS 2022 MSBuild after fixes.

Test plan

• Merge this PR into multivendor_master.
• Confirm the merge workflow _master (multivendor_master, …) jobs pass (merge + compile + DB update + custom SQL).
• In-game: open a multivendor NPC via gossip, verify the correct vendor item list appears.
• Purchase an item from a gossip-selected vendor tab; verify buy succeeds and uses the virtual vendor list.
• GM: with a multivendor window open, .npc add vendoritem <item> 0 0 0 1 and .npc delete vendoritem <item> 1 — verify items are added/removed from the active virtual vendor entry.