Fix outbox clear isolation between mediators

[thomhurst]

Summary

• scope the nonblocking background-clear semaphore to each OutboxProducerMediator instance
• preserve suppression of overlapping clears on the same mediator
• add a deterministic concurrent regression test using two independent mediators and outboxes

Root cause

The semaphore was static for each closed generic mediator type. A clear running on one mediator therefore caused another mediator with a different outbox to fail WaitAsync(TimeSpan.Zero) and silently skip its own clear.

Validation

• regression test failed before the fix on net9.0 and net10.0 (Expected: 1, Actual: 0)
• regression test passes after the fix on net9.0 and net10.0
• Core command-processor clear suite: 4 passed, 2 existing skips per target framework
• InMemory sweeper suite: 10 passed per target framework

Closes #4241

[thomhurst]

@iancooper Happy for me to merge?