[BE] openedx-core - Accept taxonomy_type on Create & Import endpoints #628

Part of #614. See parent for use case, acceptance criteria, and architecture rationale.

Depends on: data model ticket (CompetencyTaxonomy model must exist) #613

Files to modify

Key constraint

openedx_tagging must not import from openedx_learning. The dispatch to
create_competency_taxonomy() must happen at a layer that can see both packages — a
CBE-provided view mixin overriding perform_create() is one viable approach.

Example Resolution Prompt

In src/openedx_tagging/rest_api/v1/serializers.py, add taxonomy_type as a
write-only ChoiceField with required=False and choices=["tags", "competency"];
define the valid-value constants (e.g. TAXONOMY_TYPE_COMPETENCY = "competency") in a
shared location so this field and the CBE dispatch logic below stay in sync. In
src/openedx_tagging/rest_api/v1/views.py, update both perform_create() (around line
262) and the create_import() action to surface taxonomy_type from validated data —
do not call CompetencyTaxonomy directly from this layer. In
openedx_learning/applets/cbe/api.py, add create_competency_taxonomy(): call
create_taxonomy() from openedx_tagging, then create the linked CompetencyTaxonomy
row wrapped in transaction.atomic(). Wire the dispatch to
create_competency_taxonomy() at a layer that can see both packages — a CBE-provided
view mixin overriding perform_create() is one viable approach.