OpenMRS custom_validation_schema allows duplicate FSNs due to incorrect Django ORM exclude() on names relation

[filiperochalopes]

Related to: #2362 (comment)

User story

As a curator, I want OpenMRS validation on CIEL/HEAD to correctly reject duplicate Fully Specified Names so duplicates cannot be saved into the source.

Use case

Create or update a concept name in CIEL/HEAD with OpenMRS validation enabled and ensure identical FSNs are rejected.

Requirements

• Fix OpenMRS validation logic so duplicate FSNs are reliably detected and rejected.
• Ensure name comparison does not drop entire concepts from the comparison set due to unrelated names (e.g., synonyms/NULL types).
• Add a regression test reproducing the staging case where “Cerebral malaria” exists as FSN in two different concepts and confirm validation blocks it.

Acceptance criteria

• Given OpenMRS validation is enabled, when attempting to save a concept with an FSN that already exists (same locale + same text) in another active concept, then validation fails and the save is rejected.
• Validation behavior is consistent even when compared concepts have other names with excluded/NULL types.
• A regression test covers the “Cerebral malaria” duplicate scenario and passes after the fix.

More details

Check: https://github.com/OpenConceptLab/oclapi2/blob/master/core/concepts/custom_validators.py#L99

Staging evidence and context

• CIEL/HEAD on staging has custom_validation_schema = OpenMRS
• Duplicate FSN exists: “Cerebral malaria” appears on concepts 145851 and 17307337
• Suspected root cause: OpenMRS validator uses chained exclude() clauses on the reverse names relation, which can remove entire concepts from the queryset based on any related name record, preventing proper FSN comparison