fix(medcat-trainer): Handle where SOLR contains collections aren't from trainer

[alhendrickson]

This fix should sort out the CDB import in k8s. Trainer right now is failing if SOLR is used for anything else - every collection must have the field CUI in the current impl. In k8s it initialises a SOLR collection "my-collection" that doesnt have a field called CUI so it breaks it.

Right now when I save a project it hits this error.

[medcattrainer] INFO 2026-06-11 14:12:10,012 solr_utils.py l:46:Retrieving all SOLR collections: http://cogstack-solr:8983/solr/admin/collections?action=LIST
[medcattrainer] INFO 2026-06-11 14:12:10,024 solr_utils.py l:32:Retrieving solr schema: http://cogstack-solr:8983/solr/my-collection/schema
[medcattrainer] ERROR 2026-06-11 14:12:10,040 views.py l:862:Failed to search for concept_search_index. Solr Search Service not available
[medcattrainer] Traceback (most recent call last):
[medcattrainer]   File "/home/api/api/views.py", line 860, in concept_search_index_available
[medcattrainer]     return collections_available(cdb_ids)
[medcattrainer]            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[medcattrainer]   File "/usr/local/lib/python3.12/contextlib.py", line 81, in inner
[medcattrainer]     return func(*args, **kwds)
[medcattrainer]            ^^^^^^^^^^^^^^^^^^^
[medcattrainer]   File "/home/api/api/solr_utils.py", line 52, in collections_available
[medcattrainer]     _cache_solr_collection_schema_types(col)
[medcattrainer]   File "/usr/local/lib/python3.12/contextlib.py", line 81, in inner
[medcattrainer]     return func(*args, **kwds)
[medcattrainer]            ^^^^^^^^^^^^^^^^^^^
[medcattrainer]   File "/home/api/api/solr_utils.py", line 35, in _cache_solr_collection_schema_types
[medcattrainer]     cui_type = [n for n in resp['schema']['fields'] if n['name'] == 'cui'][0]['type']
[medcattrainer]                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^
[medcattrainer] IndexError: list index out of range
[medcattrainer] Internal Server Error: /api/concept-db-search-index-created/
[medcattrainer] ERROR 2026-06-11 14:12:10,042 log.py l:253:Internal Server Error: /api/concept-db-search-index-created/

The fix is to make it ignore collections that dont have the field 'cui' present.

I'll separately update the helm chart to also not make the collection, but this is the real fix here

[tomolopolis]

lgtm

[alhendrickson]

Helm chart also fixed on CogStack/cogstack-platform#97