feat: refactor group creation

[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.)

Refactors Group creation to assign the group's id, group_name, and repo_name up front in a before_validation callback, eliminating the previous "save with nil name → assign name from id → save again" pattern.

What changed:

• Added before_validation :assign_id_and_default_names, on: :create to Group. The callback reserves the next value from groups_id_seq via nextval, then uses that id to populate id/group_name/repo_name only when the caller hasn't supplied them.
• Removed the set_repo_name after_create callback. Its job is now done by the new before_validation callback.
• Converted check_repo_uniqueness from an after_create callback that raised on conflict into a standard validate :check_repo_uniqueness, on: :create that adds an error. This avoids partway-through-creation exceptions.
• Simplified the four call sites in Assignment#add_group, Assignment#add_group_api, Student#create_group_for_working_alone_student, and Student#create_autogenerated_name_group — each collapses from a four-line save/assign/save dance into a single Group.create / Group.new call.
• Re-added the NOT NULL constraint on groups.group_name in a new migration, now that the model guarantees the column is set before save. Removed the corresponding missing_non_null_constraint exception from .active_record_doctor.rb.
• Removed three tests in group_spec.rb (validate_presence_of, validate_uniqueness_of, belong_to(:course) for group_name) that are no longer testable on :create context as the callback fills nil values before validation runs. Added a comment noting that those invariants are now enforced by the callback, the unique index, and the foreign key respectively.

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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)
🎨 User interface change (change to user interface; provide screenshots)
♻️ Refactoring (internal change to codebase, without changing functionality)	x
🚦 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)

Checklist

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• I have added tests for my changes, if applicable.
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Questions and Comments

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[coveralls]

Coverage Report for CI Build 26826100410

Warning

No base build found for commit 00ab1bb on master.
Coverage changes can't be calculated without a base build.
If a base build is processing, this comment will update automatically when it completes.

Coverage: 90.314%

Details

• Patch coverage: 1 uncovered change across 1 file (12 of 13 lines covered, 92.31%).

Uncovered Changes

File	Changed	Covered	%
app/models/group.rb	11	10	90.91%
Total (3 files)	13	12	92.31%

Coverage Regressions

Requires a base build to compare against. How to fix this →

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Coverage Stats

Relevant Lines:	49961
Covered Lines:	46086
Line Coverage:	92.24%
Relevant Branches:	2175
Covered Branches:	1000
Branch Coverage:	45.98%
Branches in Coverage %:	Yes
Coverage Strength:	125.7 hits per line

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💛 - Coveralls