Fix GitOps policy install software parse function setting the wrong FMA slug field. (#46167)
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**Related issue:** Resolves #46164
The policy does parse correctly into a GitOpsPolicySpec struct, but
because there were two fields called `fleet_maintained_app_slug` it
wasn't obvious that this would happen.
# Checklist for submitter
If some of the following don't apply, delete the relevant line.
- [x] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
See [Changes
files](https://github.com/fleetdm/fleet/blob/main/docs/Contributing/guides/committing-changes.md#changes-files)
for more information.
## Testing
- [x] Added/updated automated tests
- [ ] Where appropriate, [automated tests simulate multiple hosts and
test for host
isolation](https://github.com/fleetdm/fleet/blob/main/docs/Contributing/reference/patterns-backend.md#unit-testing)
(updates to one hosts's records do not affect another)
- [x] QA'd all new/changed functionality manually
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## Summary by CodeRabbit
* **Bug Fixes**
* Fixed an issue where patch policies with different software
application slugs were incorrectly using the slug from the install
automation setting instead of the patch policy's own configured slug,
causing incorrect policy queries and validation.
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