Add clear error for Firefox / Firefox ESR conflict (#49714)

**Related issue:** Resolves #49682

Mozilla Firefox and Firefox ESR are distinct Fleet-maintained apps that
share the macOS bundle identifier `org.mozilla.firefox`, so they resolve
to one software title. Adding both to a fleet previously gave a generic
conflict error (or no error at all). This adds a clear message — "Only
one of Mozilla Firefox or Mozilla Firefox ESR can be added to the same
fleet." — on both the single-add and GitOps/batch paths. The check is
general (any two FMAs sharing a bundle identifier), with the app names
filled in dynamically.

# Checklist for submitter

- [x] Changes file added for user-visible changes in `changes/`.
- [x] Input data is properly validated, `SELECT *` is avoided, SQL
injection is prevented (using placeholders for values in statements).

## Testing

- [x] Added/updated automated tests
- [ ] QA'd all new/changed functionality manually


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **Bug Fixes**
- Prevented adding both Mozilla Firefox and Firefox ESR to the same
fleet when they share a bundle identifier.
- Updated the UI to show a specific conflict message explaining that
only one of the two can be added.
- Ensured existing workflows still work for adding new versions of the
already-selected app.
- **Tests**
- Added backend and frontend test coverage for the new conflict
detection and error-message formatting.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

changes/49682-firefox-esr-conflict-message

@@ -0,0 +1 @@
+- Added a clear error message when adding both Mozilla Firefox and Firefox ESR (which share a bundle identifier) to the same fleet, so admins understand only one of them can be added instead of seeing a generic conflict error.