**Related issue:** Resolves#48416
When adding a Fleet-maintained app, a large-installer download that's
canceled or times out now returns a clear message pointing at the likely
proxy/load-balancer timeout, instead of a raw `context canceled`.
# Checklist for submitter
- [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
- [x] QA'd all new/changed functionality manually
<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit
* **Bug Fixes**
* Improved error messages when adding Fleet-maintained apps times out or
is canceled during large installer downloads.
* Added clearer guidance for configuring server, proxy, and load
balancer timeouts.
* Properly handles additional timeout and upstream cancellation
responses, including HTTP 408, 499, and 504.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
