[FC-0118] docs: add ADR for merging similar endpoints

docs/decisions/0031-merge-similar-endpoints.rst

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+Merge Similar Endpoints
+=======================
+
+:Status: Proposed
+:Date: 2026-03-31
+:Deciders: Open edX Platform / API Working Group
+:Technical Story: Open edX REST API Standards - Consolidation of fragmented same-resource endpoints into unified parameterised views
+
+Context
+-------
+
+Open edX APIs currently expose multiple endpoints that perform closely related operations with only
+minor variations in behaviour. Rather than consolidating these into a single parameterised resource,
+the platform has grown a proliferation of narrow, action-scoped URLs — each duplicating validation,
+permission-checking, and business logic from its siblings.
+
+A prominent cluster illustrate the problem:
+
+**Certificate endpoints** (``lms/djangoapps/instructor/views/api_urls.py``):
+
+* ``enable_certificate_generation`` — enables or disables self-generated certificates for students
+* ``start_certificate_generation`` — triggers bulk certificate generation for all enrolled students
+* ``start_certificate_regeneration`` — regenerates certificates based on provided
+  ``certificate_statuses``
+
+All three are registered in ``api_urls.py`` as separate ``path()`` entries and each independently
+validates ``course_id``, checks instructor permissions, and dispatches a background Celery task —
+with near-identical boilerplate in each view.
+
+The impact of this fragmentation is felt across several dimensions:
+
+* **Redundant code**: Permission checks, serializer logic, and audit-logging are re-implemented
+  independently across views, making fixes and feature additions error-prone.
+* **Client complexity**: External systems and AI agents must discover, call, and handle errors for
+  multiple endpoints to complete a single logical workflow.
+* **Inconsistent contracts**: Divergent request/response shapes between sibling endpoints create
+  subtle integration bugs and complicate contract testing.
+
+Decision
+--------
+
+We will consolidate groups of closely related endpoints into **single, parameterised DRF views**
+(or shared service layers), using an ``action`` (or equivalent) request parameter to distinguish
+the operation being performed.
+
+Implementation requirements:
+
+* Identify endpoint groups that share the same resource domain and differ only in the operation
+  applied to that resource.
+* Expose a single URL per resource group accepting an ``action`` or ``mode`` field (or using HTTP
+  verbs semantically where REST conventions apply cleanly).
+* Move shared logic — permission checking, input validation, audit logging — into a common service
+  layer or mixin that all operations invoke.
+* Preserve backward compatibility via URL aliases or deprecation redirects for a defined transition
+  window.
+* Document the unified endpoint schema in drf-spectacular / OpenAPI, including the enumerated set
+  of valid ``action`` / ``mode`` values and their respective request/response shapes.
+
+Relevance in edx-platform
+--------------------------
+
+Confirmed fragmentation in the codebase:
+
+* **Certificate views** (``lms/djangoapps/instructor/views/api_urls.py``, lines confirmed in
+  master): The following three entries exist as separate ``path()`` registrations::
+
+      path('enable_certificate_generation', api.enable_certificate_generation,
+           name='enable_certificate_generation'),
+      path('start_certificate_generation', api.StartCertificateGeneration.as_view(),
+           name='start_certificate_generation'),
+      path('start_certificate_regeneration', api.StartCertificateRegeneration.as_view(),
+           name='start_certificate_regeneration'),
+
+Code example (target unified endpoint)
+---------------------------------------
+
+**Proposed unified certificate task endpoint**:
+
+.. code-block:: http
+
+   POST /api/instructor/v1/certificate_task/{course_id}
+   Content-Type: application/json
+
+   {
+     "mode": "generate"
+   }
+
+Valid ``mode`` values: ``generate``, ``regenerate``, ``toggle``.
+
+**Example DRF view skeleton:**
+
+.. code-block:: python
+
+   # lms/djangoapps/instructor/views/api.py
+   class CertificateTaskView(APIView):
+       """Unified entry point for certificate generation lifecycle operations."""
+
+       VALID_MODES = {"generate", "regenerate", "toggle"}
+
+       def post(self, request, course_id):
+           course_key = CourseKey.from_string(course_id)
+           _check_instructor_permissions(request.user, course_key)
+
+           mode = request.data.get("mode")
+           if mode not in self.VALID_MODES:
+               raise ValidationError({"mode": f"Must be one of: {self.VALID_MODES}"})
+
+           service = CertificateTaskService(course_key)
+           result = getattr(service, mode)(request.data)
+           return Response(result, status=status.HTTP_200_OK)
+
+Consequences
+------------
+
+Positive
+~~~~~~~~
+
+* Clients implement a single integration point per resource domain, reducing onboarding friction
+  for external systems and AI agents.
+* Shared validation, permission, and audit logic lives in one place, eliminating divergence between
+  sibling endpoints.
+* OpenAPI schemas become more compact — a single operation object per resource instead of three
+  or more.
+* Contract tests cover one endpoint per resource group, cutting test surface area without reducing
+  coverage.
+* The certificate consolidation aligns with an already-open upstream issue (#36961), increasing
+  likelihood of community acceptance.
+
+Negative / Trade-offs
+~~~~~~~~~~~~~~~~~~~~~
+
+* Existing clients calling the legacy URLs require a migration period; deprecated aliases must be
+  maintained until adoption drops sufficiently.
+* The ``mode`` / ``action`` parameter pattern diverges from strict REST conventions; teams must
+  agree on a consistent naming standard across endpoint groups.
+* A poorly designed service layer could become a "god object"; care must be taken to keep each
+  operation handler cohesive and independently testable.
+
+Alternatives Considered
+-----------------------
+
+* **Keep per-action endpoints**: Rejected. The duplication cost compounds with every new operation
+  and makes consistent error handling and logging practically impossible to enforce.
+* **Use HTTP verbs exclusively (pure REST)**: Partially applicable — ``POST`` for create,
+  ``DELETE`` for unenroll — but breaks down for operations that do not map cleanly to HTTP verbs
+  (e.g., ``enable_certificate_generation``). A hybrid approach (HTTP verbs where natural,
+  ``action`` / ``mode`` parameter otherwise) is acceptable.
+* **GraphQL mutations**: Considered but out of scope for this iteration; the platform's existing
+  REST ecosystem makes a full GraphQL migration impractical in the near term.
+
+Rollout Plan
+------------
+
+1. Implement the unified ``CertificateTaskView``; register
+   legacy paths as deprecated aliases emitting a ``Deprecation`` response header.
+2. Identify and document additional endpoint groups sharing a resource domain. Add them to the
+   placeholder table below.
+3. Announce a deprecation timeline to known API consumers and update developer documentation.
+4. Remove legacy aliases after the deprecation window closes (target: two named Open edX releases).
+
+References
+----------
+
+* Django REST Framework – Class-Based Views:
+  https://www.django-rest-framework.org/api-guide/views/