### Why is this Change Important & Necessary?
Closes Submitty#12934 

### What is the New Behavior?
If a user tries to make a new course and inputs a course code containing
characters other than lowercase letters, numbers, or underscores, they
will be redirected to the New Course page and be given a helpful error.
This validation is implemented server-side. Previously, there was no
validation for the course code field.

### What steps should a reviewer take to reproduce or test the bug or
new feature?
1. On main, sign in as instructor.
2. Go to the New Course page.
3. Ensure the term is valid and select any filesystem group.
4. Enter a valid course code (containing only lowercase letters,
numbers, and underscores) and click Create New Course.
5. You will see the popup below, and after a few seconds and a reload,
the new course appears under My Courses.

<img width="385" height="83" alt="Screenshot 2026-06-19 152318"
src="https://github.com/user-attachments/assets/8254bd1b-a223-4cd3-897d-fe6d89e53293"
/>

6. Repeat steps 2 and 3, then try to enter a course code that does not
follow the requirements given for that field
(e.g. "!", "   ", "A", "te st").
7. You will see the same message pop up as in step 5, but the course is
never made.

Now go to this PR's branch and repeat the above procedure. After
entering an invalid course code and trying to create a new course,
instead of seeing the green popup, you will be redirected to the New
Course page and will see this red popup:

<img width="395" height="73" alt="Screenshot 2026-06-24 105327"
src="https://github.com/user-attachments/assets/bdfae72e-45d7-42b3-a434-965dc0e3f123"
/>

### Automated Testing & Documentation
Introduced in issue Submitty#12951

* main: (78 commits)
  [Bugfix:TAGrading] Auto-open single file (#12625)
  [Feature:InstructorUI] Add sortable columns to manage students (#12957)
  [Dependency] Bump php-ds/php-ds from 1.7.0 to 2.0.1 in /site (#12827)
  [Refactor:TAGrading] Events Infrastructure & StatusBanner (#12941)
  [Dependency] Bump sqlalchemy from 2.0.48 to 2.0.51 in /.setup/pip (#12884)
  [Bugfix:Developer] trust hashicorp for vagrant install (#12968)
  [Feature:TAGrading] Backend for submission clustering (#12886)
  [Bugfix:InstructorUI] Missing Subsection in the Edit User form (#12955)
  [Bugfix:Submission] Warning Banners Accessibility Fix (#12956)
  [Bugfix:System] Preserve signup fields (#12939)
  [Bugfix:InstructorUI] Fix Csv Download of Subsections (#12954)
  [Bugfix:InstructorUI] Fix Manage Students Toggle Columns (#12953)
  [Feature:TAGrading] Persistent Auto-Open (#12931)
  [Bugfix:Submission] Dot File Upload (#12924)
  [Refactor:System] Declutter workers (#12815)
  [Bugfix:Developer] save/restore autograding_containers.json (#12952)
  [Bugfix:InstructorUI] Create Course Validation (#12949)
  [Testing:Notifications] Fix grade_inquiries.spec.js (#12950)
  [UI/UX:System] Add fullscreen button vue component (#12936)
  [Bugfix:TAGrading] Student Name Resize (#12946)
  ...

# Conflicts:
#	.pylintrc