Ticket created from upstream issue: https://github.com/389ds/389-ds-base/issues/7032

*Issue Description*
With the commit https://github.com/389ds/389-ds-base/commit/8b6b3a9f9a5ab2cd087e96be1a972c0ba20a2264#diff-0b5030c27013744647254b2b3e798b5cac7135438425a4e39509e85e02109a44 the check for indexes now raises a CRITICAL issue in IdM deployments.

*Package Version and Platform:*

• Platform: Fedora 42
• Package and version: 389-ds-base-3.1.3.202510031754git13d8cc356-1.fc42.x86_64
  (installed from the copr repo @389ds/389-ds-base-nightly)

*Steps to Reproduce*
Steps to reproduce the behavior:
1. install ipa server and ipa-healthcheck packages
2. configure the server with `ipa-server-install --domain ipa.test --realm IPA.TEST --setup-dns --forwarder 10.11.5.160 -a Secret123 -p Secret123 -U`
3. run healthcheck: `/usr/bin/ipa-healthcheck --source ipahealthcheck.ds.backends --check BackendsCheck`

```

1. python3 /usr/bin/ipa-healthcheck --source ipahealthcheck.ds.backends --check BackendsCheck
   [
   
   Unknown macro: { "source"}

   ,
   

   Unknown macro: { "source"}

   ,

   Unknown macro: { "source"}

   ]
   ```

*Expected results*
No error should be reported on new installation

The error seems to happen because of a missing key:
https://github.com/389ds/389-ds-base/blob/8b6b3a9f9a5ab2cd087e96be1a972c0ba20a2264/src/lib389/lib389/backend.py#L706

pdb shows that we get a KeyError:
```
(Pdb) index
<class 'lib389.index.Index'>(instance_name="IPA-TEST", dn="entryrdn")
(Pdb) n
> /usr/lib/python3.13/site-packages/lib389/backend.py(706)_lint_system_indexes()
-> expected_scanlimit = expected_config['scanlimit']
(Pdb) n
KeyError: 'scanlimit'
> /usr/lib/python3.13/site-packages/lib389/backend.py(706)_lint_system_indexes()
-> expected_scanlimit = expected_config['scanlimit']
(Pdb) expected_config

(Pdb) w
```

The code could use `expected_scanlimit = expected_config.get('scanlimit')` instead as the call returns None if there is no value with this key in the dictionary.