Mismatch between charm channel version and snap channel version

## Steps to reproduce
Install charm mongodb channel 6/stable
Check the installed snap version

## Expected behavior
That the channel used in the charm matches the channel used in the snap

## Actual behavior
```bash
# juju status | grep mongodb
mongodb active 1 mongodb 6/stable 199 yes
```

```bash
juju ssh mongodb/0 -- snap list | grep mongo
charmed-mongodb 6.0 123 6/edge canonical✓ held
```

## Versions
Operating system: Ubuntu Jammy
Juju: 3.6
Charm revision: 6/stable rev 199

Start by reproducing the mismatch with the Juju status and snap list commands in the issue, then trace how the MongoDB charm's 6/stable channel selects the snap channel. Done means installing charm revision 199 from 6/stable results in the expected matching snap channel.

Written by the indexing model from the issue text.