fix: missing null and presence checks

[YheChen]

Proposed Changes

*(Describe your changes here. Also describe the motivation for your changes: what problem do they solve, or how do they improve the application or codebase? If this pull request fixes an open issue, use a keyword to link this pull request to the issue.)*Address the active_record_doctor missing_non_null_constraint and missing_presence_validation checks:

active_record_doctor's missing_non_null_constraint and missing_presence_validation
checks flagged a number of columns where the database schema and the model-level
expectations had drifted apart — columns the models treat as required but that the
database still allowed to be NULL, and vice versa. This PR closes those gaps and fixes
the bugs that the new constraints surfaced.

Schema and model changes:

• Added a migration setting NOT NULL on columns the models already treat as required
  (timestamps, belongs_to foreign keys, presence-validated columns).
• Added presence/inclusion/exclusion validators to models for NOT NULL columns that
  lacked them. Booleans use inclusion: [true, false]; columns defaulting to "" / {}
  use exclusion: [nil].
• Marked Annotation/AnnotationText belongs_to :creator as optional, since the
  column is nullified when a Role is destroyed.

Bugs surfaced by the new constraints:

• Group creation flows were saving group_name as NULL before renaming.
• Criterion/GradeEntryStudent#randomly_assign_tas inserted rows with a null ta_id
  when given no TAs.
• SplitPdfJob now sets split_pdf_log_id when upserting SplitPage rows, which the
  new NOT NULL constraint requires.

Screenshots of your changes (if applicable)

Associated documentation repository pull request (if applicable)

Type of Change

(Write an X or a brief description next to the type or types that best describe your changes.)

Type	Applies?
🚨 Breaking change (fix or feature that would cause existing functionality to change)
✨ New feature (non-breaking change that adds functionality)
🐛 Bug fix (non-breaking change that fixes an issue)	X
🎨 User interface change (change to user interface; provide screenshots)
♻️ Refactoring (internal change to codebase, without changing functionality)
🚦 Test update (change that only adds or modifies tests)
📦 Dependency update (change that updates a dependency)
🔧 Internal (change that only affects developers or continuous integration)	X

Checklist

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[coveralls]

Coverage Report for CI Build 26544307572

Coverage increased (+0.002%) to 90.279%

Details

• Coverage increased (+0.002%) from the base build.
• Patch coverage: 66 of 66 lines across 34 files are fully covered (100%).
• 5 coverage regressions across 1 file.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

5 previously-covered lines in 1 file lost coverage.

File	Lines Losing Coverage	Coverage
app/controllers/grade_entry_forms_controller.rb	5	79.87%

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Coverage Stats

Relevant Lines:	49943
Covered Lines:	46059
Line Coverage:	92.22%
Relevant Branches:	2151
Covered Branches:	971
Branch Coverage:	45.14%
Branches in Coverage %:	Yes
Coverage Strength:	121.69 hits per line

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💛 - Coveralls