Suggest: do not embed the child listing of folderish suggestions (#105)

[reekitconcept]

Fixes #105.

What

SolrSuggest.serialize_brain() serializes a small set of portal types in full
via ISerializeToJson rather than from the catalog brain. For any Dexterity
container, plone.restapi resolves that to SerializeFolderToJson, which
defaults to include_items=True. So for a folderish suggest type, every single
suggestion:

1. ran an extra catalog query over that object's children, and
2. carried the complete child listing (items, items_total, batching) in
   the response.

Nothing in the suggest UI consumes those keys — it renders a title, a type and
a link.

Why it matters

Suggest is a type-ahead endpoint: it fires on nearly every keystroke, and
the cost multiplies by keystrokes x suggestions. It degrades exactly the
interaction that is supposed to feel instant, and it does so silently — no
error, no log entry, just a fat response and extra catalog work per request.

The Member branch is not dead code; the frontend has a matching render branch
in SolrSearchWidget.jsx. And folderish suggest types are not exotic:
collective.person's Person is a Container (it accepts File and
Image children), so a person-directory intranet that surfaces people in the
suggest dropdown lands here directly, with the payload growing with each
attachment on each person.

How

Pass include_items=False explicitly. Plain SerializeToJson accepts and
ignores the keyword, so the call stays correct for both folderish and
non-folderish types — no type check needed, and non-folderish types are
completely unaffected.

The inline ["Member"] literal is lifted into a module-level
FULL_SERIALIZATION_TYPES constant. That names the behaviour, and it lets the
test cover the folderish path without adding a Member content type to the
fixtures.

Tests

New backend/tests/services/suggest/test_suggest_serialize_brain.py, three
cases. They exercise serialize_brain() directly and only request the
integration fixture, so they need no running Solr despite living under
tests/services/suggest/.

Document stands in for the folderish suggest type: with plone.volto applied
it is a Container, so it genuinely routes through SerializeFolderToJson —
the first assertion (is_folderish is True) pins that down, so the
"no items" assertions cannot pass vacuously.

Verified the test actually catches the bug: reverting just the
include_items=False argument makes test_full_serialization_omits_the_child_listing
fail on assert "items" not in data. With the fix, all three pass.

Note / known limitation

SerializeFolderToJson lets the request override the keyword:

include_items = self.request.form.get("include_items", include_items)

So /@solr-suggest?query=x&include_items=true still returns the items. That is
a caller opting in explicitly rather than an accidental default, so this PR
does not defend against it — recording it here so it is not a surprise later.

Scope

Small and self-contained; no API change for any existing consumer. Intended for
the batch of small fixes going into the upcoming release candidate — please
review independently, not to be merged as part of anything else.