[michaelstaib]

Adds tests for execution group members dropped by a non-null violation. A dropped member is never registered, and the task that drops it releases the member's group slot when its turn ends, so later members still run.

Changes:

• ExecutionGroupDroppedMemberTests count the resolver tasks returned to the pool:
  • A later member still runs, and every task goes back to the pool, when the non-null parent of an earlier member is nulled.
  • The request completes, and every task goes back to the pool, when a non-null group root is nulled after its members were created.

No product code changes.

Stacked on #138.

Tests: the first test hangs if the slot release for unregistered children is removed. Execution.Tests pass on net10.0 and net8.0.

[Copilot]

🟢 Approval recommended

No active review issues are recorded for this test-only change adding regression coverage for dropped execution-group members and task cleanup.

No open issues 🎉

What changed in this PR

Adds regression tests for execution-group members dropped by non-null violations, extending the child-task cleanup coverage from #138. No product code changes.

Changes:

• Verifies that a later group member still runs when an earlier member is dropped.
• Verifies request completion when a non-null group root is nulled after member tasks are created.
• Counts resolver-task rentals and returns to check that all tasks return to the pool.

File	Description
src/​HotChocolate/​Core/​test/​Execution.Tests/​ExecutionGroups/​ExecutionGroupDroppedMemberTests.cs	Adds two dropped-member regression tests and a counting resolver-task pool.

_{Review effort: Balanced}