interfaces: MinerImpl::chainman(): Assertion 'm_node.chainman' failed.

The following assertion may fail during shutdown:

```
node/interfaces.cpp:1057 ChainstateManager &node::(anonymous namespace)::MinerImpl::chainman(): Assertion `m_node.chainman' failed.
```

Debug logs for the node that hit the assertion as well as the full test logs can be found [here](https://drive.google.com/file/d/1HWiL9h0ZjapdesjZix1UpOq_RME9jhiT/view).

_This was found with a test running on [Antithesis](https://antithesis.com/) (see [this gist](https://gist.github.com/dergoegge/2f66365264f316b5ca60ec7565d06d74) for context on how we use it)._

Start at node/interfaces.cpp:1057, where MinerImpl::chainman() asserts during shutdown, then review the shutdown-related logs and full test logs linked in the issue. Trace the shutdown sequence to identify why m_node.chainman is unavailable; done means the assertion no longer occurs during shutdown and the relevant test still passes.

Written by the indexing model from the issue text.