[08.6] [M0.7] Make vendor purchases fully atomic

[alseif0x]

Parent: #20 (D-C8).

Current 3.4.3 already commits ordinary item purchases as one transaction after PR #107, but the currency-vendor branch still publishes currency gain/costs into session runtime before awaiting commit_transaction. A cancelled handler or an unknown COMMIT result can therefore leave runtime and durable currency/item-turn-in state inconsistent.

C++ anchors:

• src/server/game/Handlers/ItemHandler.cpp::HandleBuyItemOpcode
• src/server/game/Entities/Player/Player.cpp::BuyItemFromVendorSlot
• _StoreOrEquipNewItem / TakeExtendedCost

Rust targets:

• crates/wow-world/src/handlers/character.rs::handle_buy_item
• existing player-money persistence/cancellation fence in crates/wow-world/src/session.rs
• crates/wow-database/src/transaction.rs

Done:

• Plan currency gains, currency costs, item turn-ins and item/gold mutations without publishing runtime state before durable success.
• Commit every purchase-owned mutation atomically.
• Preserve runtime state after a definite rollback.
• Treat cancelled or unknown COMMIT outcomes as indeterminate and require reload without allowing a stale full save to overwrite the transaction.
• Focused positive, rollback, and unknown-outcome tests; C++ refs documented.
• Bot/client capture-diff evidence for a representative vendor purchase before merge.

Closes the D-C8 slice of #20; it does not expand vendor validation/content parity beyond the existing handler.