feat: implement instructor API v2 grading GET endpoints

[brianjbuck-wgu]

Summary

This PR implements three new GET endpoints for the Instructor Dashboard API v2 grading operations, as specified in the OpenAPI schema from #37743.

New Endpoints:

1. GET /api/instructor/v2/courses/{course_id}/learners/{email_or_username}

   • Retrieves comprehensive learner information including profile, enrollment status, progress URLs, and current grading data
   • Accepts either username or email as identifier

2. GET /api/instructor/v2/courses/{course_id}/problems/{location}

   • Retrieves problem metadata with display name and course hierarchy breadcrumbs
   • Helps instructors navigate and identify specific problems within course structure

3. GET /api/instructor/v2/courses/{course_id}/tasks/{task_id}

   • Checks status of background tasks (e.g., bulk grading operations)
   • Returns task state (pending/running/completed/failed) with progress tracking and results

All endpoints follow existing instructor API v2 patterns with proper authentication, permission checks, and error handling.

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