perf: speed up the CCX grade report and fix its CSV output

[sergivalero20]

Description

This PR improves the synchronous CCX coach grade report (ccx_grades_csv, CCX Coach dashboard → Student Admin → "Download student grades"). It addresses three independent problems, one per commit:

1. Performance — N+1 read of persisted grades (perf)

ccx_grades_csv iterated enrolled learners with CourseGradeFactory().iter() without bulk-prefetching persisted course/subsection grades, so each student triggered its own database read. Grade-related query count grew linearly with
enrollment, and because the report is synchronous it holds a web worker for the full duration, slow, and prone to timing out on larger classes.

The fix reuses the exact pattern the asynchronous CourseGradeReport already relies on (CourseGradeReport._rows_for_users / _CourseGradeBulkContext): call prefetch_course_and_subsection_grades() once for the whole class and wrap the iteration in modulestore().bulk_operations(). This commit is output-preserving — the CSV is byte-for-byte identical, only faster.

2. Bug — bytes repr in the CSV (fix)

student.email.encode('utf-8') / student.username.encode('utf-8') return bytes on Python 3, and csv.writer serializes bytes with repr(), so the downloaded file contained values like b'learner@example.com' and b'learner'. The .encode() calls were a Python 2 leftover — csv.writer handles unicode natively. This changes the CSV output (it becomes correct).

3. Usability — generic download filename (feat)

Content-Disposition was a bare attachment with no filename, so every CCX downloaded as ccx_grades.csv, making it impossible to tell classes or download dates apart. It now uses course_filename_prefix_generator() to build <course_prefix>_grade_report_<YYYY-MM-DD-HHMM>.csv, matching the naming convention already used by CourseGradeReport downloads. This changes the downloaded filename.

Supporting information

Fixes #38911

Related existing implementation reused by this PR:

• lms/djangoapps/instructor_task/tasks_helper/grades.py —
  CourseGradeReport._rows_for_users, _CourseGradeBulkContext
• lms/djangoapps/grades/api — prefetch_course_and_subsection_grades
• common/djangoapps/util/file.py — course_filename_prefix_generator

Testing instructions

Automated

Run the CCX grade report tests:

pytest lms/djangoapps/ccx/tests/test_views.py -k test_grades_csv \
  --ds=lms.envs.test --nomigrations --reuse-db

And the full CCX view test module, to confirm nothing else regressed:

pytest lms/djangoapps/ccx/tests/test_views.py \
  --ds=lms.envs.test --nomigrations --reuse-db

Two tests cover this change:

• test_grades_csv — extended to assert the new filename pattern and that email/username are written as plain text (the fixture includes a learner with a unicode username, the case that motivated the original .encode()). The existing grade-breakdown assertions are unchanged, which demonstrates the performance commit does not alter output.
• test_grades_csv_prefetches_persisted_grades — new; asserts a single bulk prefetch is issued for the CCX covering every enrolled learner (locks in the N+1 fix without brittle exact query counts).

Quality gates:

make quality

Manual

Setup: enable CCX (FEATURES['CUSTOM_COURSES_EDX'] = True), create a CCX, and enroll several learners who have attempted graded content.

Output correctness

1. As the CCX coach, go to the CCX Coach dashboard → Student Admin → Download student grades.
2. Verify the file downloads as <course_prefix>_grade_report_<timestamp>.csv (previously the generic ccx_grades.csv).
3. Open the CSV and verify the email and username columns contain plain values (previously b'learner@example.com'). Include a learner whose username contains non-ASCII characters.
4. Verify the grade columns and the overall grade are identical to what the report produced before this PR.

Performance

5. Enable SQL logging in your devstack LMS settings:

   LOGGING['loggers']['django.db.backends'] = {
       'level': 'DEBUG', 'handlers': ['console'], 'propagate': False,
   }

   (Django Debug Toolbar also works if you have it enabled.)

6. Download the report and note the number of grade-related queries.

7. Enroll additional learners in the same CCX and download again. The grade-related query count should stay essentially flat rather than growing with the number of learners. Before this PR it grew linearly.

Edge cases

8. A learner enrolled but who has not attempted any graded content still appears in the report, with zero grades (unchanged behavior).
9. A CCX with no active enrollments returns a valid, empty CSV without error.
10. Content hidden from learners (visible_to_staff_only) remains excluded from the grade columns, as before.

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