Add test coverage for WeakGc::new_in constructor

[mrhapile]

This PR adds a unit test covering the WeakGc constructor.

Currently, oscars/src/collectors/mark_sweep/pointers/weak.rs has
0% test coverage. The WeakGc::new_in constructor is the only
executable code in the file but is not exercised by existing tests.

This PR introduces a minimal test verifying that a WeakGc can be
created from a strong Gc reference. Running this test executes
all lines inside WeakGc::new_in.

Coverage impact:

Before:
weak.rs → 0%

After:
weak.rs → ~100%

No changes to GC implementation are introduced. This PR only
adds test coverage for existing behavior.

[Copilot]

Pull request overview

Adds a minimal unit test to exercise WeakGc::new_in so oscars/src/collectors/mark_sweep/pointers/weak.rs is covered by the mark-sweep collector test suite.

Changes:

• Add a new weak_gc_tests module with a test that constructs a WeakGc from a strong Gc.
• Execute WeakGc::new_in in tests to eliminate previously uncovered lines in weak.rs.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

You can also share your feedback on Copilot code review. Take the survey.

[mrhapile]

[mrhapile]

@nekevss can u give a quick review : )