Free Merchant Caravans don't own items in their truck and refuse to trade them

### Describe the bug

The Free Merchant caravans that spawn as part of random encounters don't own any of the items within their truck, nor do those items appear when interacting with the trade interface for the caravan master. You can also take these items without consequences in and of themselves; the merchants will still get mad for climbing in the bed, though.

### Attach save file

[Actual Test World-trimmed.tar.gz](https://github.com/CleverRaven/Cataclysm-DDA/files/13968209/Actual.Test.World-trimmed.tar.gz)

### Steps to reproduce

1. Go to a public washroom seven or more days after the Cataclysm.
2. Find the Free Merchant caravan.
3. Talk with the caravan master.
4. Initiate trade.
5. See that none of the items within the truck bed are available.

### Expected behavior

I expect that taking the items in the bed of the truck will be counted as stealing, and that the Free Merchants will be willing to sell them to me.

### Screenshots

N/A

### Versions and configuration

- OS: Windows
- OS Version: 10.0.22621.3007 (22H2)
- Game Version: 1215fbe [64-bit]
- Graphics Version: Tiles
- Game Language: System language []
- Mods loaded: [
Dark Days Ahead [dda],
Disable NPC Needs [no_npc_food],
Portal Storms Ignore NPCs [personal_portal_storms],
Slowdown Fungal Growth [no_fungal_growth],
Personal Test Data [personal_test_data]
]

### Additional context

The Personal Test Data mod is just something I use for personal projects and testing; I haven't made any changes to the Free Merchant caravans with it.

Start by loading the attached save file and following the listed caravan trade steps to reproduce the missing items. Trace how Free Merchant caravan contents are assigned ownership and exposed through the trade interface. Done means truck-bed items are offered for trade and taking them is treated as stealing.

Written by the indexing model from the issue text.