Downloads: `Unable to save episode, have you run out of space?` error

### Description

We have a user reporting that they have over 70GB of free space, but keep seeing an error: `Unable to save episode, have you run out of space?`

We noticed in the logs:
```
DownloadManager: Failed to copy downloaded file from location: to destination: error:

```

As per our internal discussion, this needs a closer look: p1734167162828969-slack-C02A333D8LQ

> with the info we added it looks there is some issue of having duplicate files in the destination folder. so when the download finish in the temporary location when we try to move the file to the permanent location it fails[...]

### Step-by-step reproduction instructions

n/a

### Screenshots or screen recording


### Did you search for existing bug reports?

- [x] I have searched for existing bug reports.

### Device, Operating system, and Pocket Casts app version

App Version: 7.80
Device: iPhone14,3
OS: 18.3

Start by tracing DownloadManager's copy from the temporary download location to the permanent destination, using the logged failure and the duplicate-file suspicion as the investigation focus. Reproduce the save failure if possible on the listed iPhone and iOS versions; done means identifying and resolving the cause of the false out-of-space error, with a verified download result.

Written by the indexing model from the issue text.