feat(upload): test data generator for PDP, AR, and course files

Adds operations/generate_test_data.py which creates synthetic Bishop State
CSV fixtures in data/test_uploads/ for upload pipeline testing: PDP cohort AR
(500 rows, 90 cols), cohort submission (500 rows, 35 cols), course AR (5000
rows, 39 cols), ML predictions (2500 rows), bad headers, mixed casing, and a
~62MB oversized file. Also adds data/test_uploads/ to .gitignore.

Author: William-Hill

.gitignore

@@ -183,3 +183,6 @@ operations/convert_institution_id_to_string.py
 operations/verify_institution_id.py
 .vercel
 .env.deploy
+
+# Test upload fixtures (generated — do not commit)
+data/test_uploads/