bugfix: random assignment after grader filtering

[YheChen]

Proposed Changes

Ensure random grader assignment uses only the TA IDs accepted by Grouping.assign_tas.

Previously, randomly_assign_tas calculated assignment counts from the original submitted IDs and ignored the filtered TA IDs yielded by assign_tas. A filtered-out role, such as a student, could therefore be reintroduced into the generated membership pairs. Its requested weight also remained part of the workload calculation.

This PR:

• preserves each submitted weight by role ID;
• calculates the total and assignment counts after assign_tas filters the IDs;
• constructs membership pairs using only the filtered IDs;
• returns no assignments when no eligible weighted TAs remain; and
• adds a regression test confirming that an invalid role with a larger weight is excluded and the valid TA receives all selected groups.

Valid random-assignment requests retain the existing relative weighting behavior. This is a focused follow-up to the review discussion in #8060 and is independent of #8072.

Screenshots of your changes (if applicable)

Type of Change

Type	Applies?
🚨 Breaking change
✨ New feature
🐛 Bug fix	X
🎨 User interface change
♻️ Refactoring
🚦 Test update
📦 Dependency update
📖 Documentation update
🔧 Internal

Checklist

Before opening this pull request:

• I have performed a self-review of my changes.
• I have added tests for my changes.
• I have updated the project documentation, if applicable.
• If this is my first contribution, I have added myself to the list of contributors.

After opening this pull request:

• I have updated the project Changelog.
• I have verified that the pre-commit.ci checks have passed.
• I have verified that the CI tests have passed.
• I have reviewed the test coverage changes reported by Coveralls.
• I have requested a review from a project maintainer.

Questions and Comments

[coveralls]

Coverage Report for CI Build 29963842412

Coverage increased (+0.001%) to 90.415%

Details

• Coverage increased (+0.001%) from the base build.
• Patch coverage: 13 of 13 lines across 2 files are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	51302
Covered Lines:	47400
Line Coverage:	92.39%
Relevant Branches:	2470
Covered Branches:	1218
Branch Coverage:	49.31%
Branches in Coverage %:	Yes
Coverage Strength:	127.86 hits per line

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💛 - Coveralls

[david-yz-liu]

Nice work, @YheChen!