done_callback raises RuntimeError during interpreter shutdown

**Describe the issue**:

`done_callback` in client.py raises `RuntimeError: cannot schedule new futures after interpreter shutdown` when futures complete during interpreter shutdown.

`Future.add_done_callback()` schedules callbacks via `Future._cb_executor` (a class-level `ThreadPoolExecutor`). During shutdown, CPython's `concurrent.futures.thread._python_exit()` atexit handler kills all `ThreadPoolExecutor` instances and sets a module-level `_shutdown = True` flag. The Tornado IOLoop daemon thread is still alive draining `done_callback` coroutines — when they call `_cb_executor.submit()`, it raises `RuntimeError`.

The fix is to catch shutdown-related `RuntimeError` in `done_callback` since the callback is no longer meaningful if the interpreter is shutting down.

**Minimal Complete Verifiable Example**:

```python
import time
from distributed import Client, LocalCluster

def on_done(future):
print(f"Callback: {future.key}")

if __name__ == "__main__":
cluster = LocalCluster(n_workers=2, threads_per_worker=1)
client = Client(cluster)

# pure=False ensures unique keys; staggered sleeps so futures complete during shutdown
futures = client.map(time.sleep, [0.2 * i for i in range(20)], pure=False)
for f in futures:
f.add_done_callback(on_done)

# Exit while futures are still completing — don't call client.close()
time.sleep(1.5)
```

**Anything else we need to know?**:

The error path is: `done_callback` → `callback(future)` → `_cb_executor.submit(execute_callback)` → `RuntimeError`.

Even creating a new `ThreadPoolExecutor` would not help because CPython's `concurrent.futures.thread.submit()` checks both the instance flag (`self._shutdown`) and the module-level flag (`_shutdown`), which was already set by `_python_exit()`.

Suggested fix in `done_callback`:

```python
async def done_callback(future, callback):
while future.status == "pending":
await future._state.wait()
try:
callback(future)
except RuntimeError as e:
if "shutdown" not in str(e) and "interpreter" not in str(e):
raise
```

This is also a prerequisite for a related `dask-gateway` bug where `cleanup_lingering_clusters` atexit fails to send HTTP DELETE to shut down clusters because the same executor shutdown blocks aiohttp's DNS resolution.

**Environment**:

- Dask version: 2025.9.1
- Python version: 3.11.14
- Operating System: macOS 15.7.4 (arm64)
- Install method: uv (pip)