Add test case for failure in search call

[tiberiuichim]

For some reason the @search endpoint fails when passed with expanders like:

@search?expand=navigation,actions&expand.navigation.depth=2

I'm not sure how who's trying to use this URL and for what purpose, I saw it as an error in our logs.

---

📚 Documentation preview 📚: https://plonerestapi--1898.org.readthedocs.build/

[mister-roboto]

@tiberiuichim thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

[tiberiuichim]

Tests fail with:

Error in test test_expand (plone.restapi.tests.test_search_utils.TestUnflattenDottedDict)
Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/unittest/case.py", line 60, in testPartExecutor
    yield
  File "/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/unittest/case.py", line 676, in run
    self._callTestMethod(testMethod)
  File "/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/unittest/case.py", line 633, in _callTestMethod
    method()
  File "/home/runner/work/plone.restapi/plone.restapi/src/plone/restapi/tests/test_search_utils.py", line 31, in test_expand
    unflatten_dotted_dict(dct),
  File "/home/runner/work/plone.restapi/plone.restapi/src/plone/restapi/search/utils.py", line 35, in unflatten_dotted_dict
    inner = reduce(create_or_get, [result] + key_segments[:-1])
  File "/home/runner/work/plone.restapi/plone.restapi/src/plone/restapi/search/utils.py", line [28](https://github.com/plone/plone.restapi/actions/runs/13998116091/job/39198164883?pr=1898#step:6:29), in create_or_get
    return dct.setdefault(key, {})
AttributeError: 'str' object has no attribute 'setdefault'

[davisagli]

The search endpoint doesn't support expanders. The use of them here suggests that maybe the frontend is trying to call the @search endpoint as a content route when it shouldn't.

The use of unflatten_dotted_dict here is meant for deserializing structured queries like path.depth=1. I'll adjust it to raise a BadRequest instead of an AttributeError if it encounters a key that was already set to a string. On second thought, I'll make it transfer the str value to the "query" key of the new dict, which is what actually makes sense in the context of search queries.