fix(Core/Vehicle): no longer eject and overwrite an existing passenger

[sogladev]

Changes Proposed:

This PR proposes changes to:

• Core (units, players, creatures, game systems).
• Scripts (bosses, spell scripts, creature scripts).
• Database (SAI, creatures, etc).

followup on:

• fix(Scripts/Northrend): Update Drakkensryd proto-drake vehicle logic #25587

TODO: Using the harpoon can overwrite / mount an already occupied player's seat.

Current vehicle logic allows a player to board a vehicle by ejecting an existing passenger. It's more sensible to eject a passenger before overwriting their seat.

Unsure which core logic or scripts rely on this force-eject behavior.

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[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adjusts vehicle boarding logic to avoid forcibly ejecting an existing passenger when attempting to add a new passenger to an occupied seat.

Changes:

• When the requested seat is occupied, search for the next empty seat instead of ejecting the current occupant.
• If no empty seat exists (or the computed seat cannot be found), fail the add operation.

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[Copilot]

The code switches the seat iterator to nextSeatId, but does not update seatId. If later logic in AddPassenger uses the seatId parameter (e.g., for packets, bookkeeping, or aura/vehicle state), this can desync the selected seat from the recorded seat id. Fix by updating seatId = nextSeatId; (or consistently using seat->first from this point onward) so the chosen seat id and iterator remain aligned.

Suggested change

	seatId = nextSeatId;

[Copilot]

Using a raw -1 sentinel for an invalid seat id makes the intent harder to read and duplicates knowledge about GetNextEmptySeat’s contract. Prefer a named constant (e.g., INVALID_SEAT_ID) or an std::optional<int8>-style pattern (if feasible in this codebase) to make the failure condition self-documenting.

[Copilot]

The PR description says it's more sensible to eject a passenger before overwriting their seat, which implies preserving a force-eject flow but fixing ordering. The code change instead removes ejection entirely and fails when no empty seats remain. Please update the PR description to match the implemented behavior (or adjust the implementation if the intended behavior is still to allow boarding by ejecting under some conditions).