Improve apple MDM parsing (#47344)

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**Related issue:** Resolves #

# 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.

- [x] Input data is properly validated, `SELECT *` is avoided, SQL
injection is prevented (using placeholders for values in statements), JS
inline code is prevented especially for url redirects, and untrusted
data interpolated into shell scripts/commands is validated against shell
metacharacters.
- [x] Timeouts are implemented and retries are limited to avoid infinite
loops
- [x] If paths of existing endpoints are modified without backwards
compatibility, checked the frontend/CLI for any necessary changes

## Testing

- [x] Added/updated automated tests
- [x] 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

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

* **New Features**
* Enforced request body size limit for Apple MDM operations (≈16 MiB
cap).

* **Improvements**
* Safer Apple MDM plist parsing with bounds and complexity checks to
reject malformed/oversized payloads.
* Decoder updated to more strictly accept XML check-in/command payloads.

* **Tests**
* Added unit tests covering bounded plist decoding and XML-only decoding
behavior.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

changes/mdm-apple-plist-parsing

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+* Improved Apple MDM enrollment and device management request parsing