inspector: properly shut down uv_async_t

(Split out from #30467 because that one is under debate)

Closing in the Agent destructor is too late, because that happens when the Environment is destroyed, not when libuv handles are closed.

This fixes a situation in which the same libuv loop is re-used for multiple Environment instances sequentially, e.g. in our cctest.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines