feat: add learner intent xpert api endpoint

[brobro10000]

Description:

Adds the first Learner Pathways Xpert-backed API endpoint for deriving structured learning intent from learner input.

What changed

• Added Learner Pathways request/response serializers for learning intent.
• Added LearnerPathwaysViewSet.learning_intent.
• Registered the /api/v1/learner-pathways/learning-intent route.
• Wired the endpoint to the reusable BasePromptViewSet helpers for:
  • request validation
  • prompt lookup
  • system prompt construction
  • Xpert message creation
  • Xpert execution
  • response content extraction
  • JSON parsing
• Added route-level auth, permission, and scoped throttling.
• Added learner_pathways_learning_intent throttle rate.
• Added endpoint tests for serializer validation, routing, auth, throttling, happy path behavior, response passthrough, and failure cases.

Jira:
ENT-11858

Merge checklist:

• ./manage.py makemigrations has been run
  • Note: This must be run if you modified any models.
    • It may or may not make a migration depending on exactly what you modified, but it should still be run.

Post merge:

• Ensure that your changes went out to the stage instance
• Deploy to prod instance

[codecov]

Codecov Report

❌ Patch coverage is 93.87755% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.51%. Comparing base (0854543) to head (c2f31ab).

Files with missing lines	Patch %	Lines
...se_access/apps/api/serializers/learner_pathways.py	77.77%	4 Missing ⚠️
enterprise_access/apps/prompts/api/__init__.py	95.74%	1 Missing and 1 partial ⚠️

❌ Your patch check has failed because the patch coverage (93.87%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #212      +/-   ##
==========================================
- Coverage   86.52%   86.51%   -0.01%     
==========================================
  Files         154      156       +2     
  Lines       12920    12970      +50     
  Branches     1233     1234       +1     
==========================================
+ Hits        11179    11221      +42     
- Misses       1425     1430       +5     
- Partials      316      319       +3     

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[pwnage101]

Nit: use modern pytest-style assertions, e.g.:

Suggested change

	self.assertIn('learner_pathways_learning_intent', rates)
	assert 'learner_pathways_learning_intent' in rates

[brobro10000]

inform: I have updated the PR, but, there are over 100+ instances of assertIn as a validation approach spanning features like checkout intent, customer billing, provisioning, subsidy access policy, checkout intent/checkout bff and browse and request whereas the modern approach is used sparingly in comparison.

Should there be a seperate initiative via a ticket to modernize the testing prose documented in a separate ticket?

[brobro10000]

As a followup, the skills may also need to be updated to avoid legacy/non-modern approaches to test writing