bugfix: cross-assignment grader membership changes

[YheChen]

Proposed Changes

Restrict grader assignment and unassignment operations to groupings belonging to the requested assignment.

Previously, Grouping.assign_tas validated grouping IDs with a global query. A grouping from another assignment could therefore be included in a bulk assignment. Grouping.unassign_tas similarly deleted TA memberships using only their membership IDs, so a membership from another assignment could be removed.

This PR:

• scopes grouping IDs in assign_tas through the supplied assignment;
• scopes grouping IDs in unassign_tas through the supplied assignment;
• restricts membership deletion by both membership ID and validated grouping ID; and
• adds regression tests that mix valid and cross-assignment records, confirming valid operations still succeed while cross-assignment changes are ignored.

This is a focused follow-up to the review discussion in #8060. Random grader assignment and weighting behavior are unchanged.

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 29941583894

Warning

Build has drifted: This PR's base is out of sync with its target branch, so coverage data may include unrelated changes.
Quick fix: rebase this PR. Learn more →

Coverage increased (+0.003%) to 90.414%

Details

• Coverage increased (+0.003%) from the base build.
• Patch coverage: 17 of 17 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:	51296
Covered Lines:	47394
Line Coverage:	92.39%
Relevant Branches:	2470
Covered Branches:	1218
Branch Coverage:	49.31%
Branches in Coverage %:	Yes
Coverage Strength:	127.85 hits per line

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💛 - Coveralls

[david-yz-liu]

Looks good, thanks @YheChen!