Fix default text processing provider in AI settings

[julien-nc]

When no AI setting has been set, make sure the selected text processing provider is the same as the one that will be actually used by the manager.

The manager takes the first available provider from its getProviders method result. The settings were showing the last in this list.

refs nextcloud/text#5269 (comment)

Also, add provider name in results of fake providers (in the testing app).

I didn't check but we might have the same problem for STT, text2image and translation.

[julien-nc]

/backport to stable28

[backportbot]

The backport to stable28 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable28
git pull origin stable28

# Create the new backport branch
git checkout -b backport/43993/stable28

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick ba94de25

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/43993/stable28

Error: No changes found in backport branch

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.