kvserver/rangefeed: StreamManager.NewStream fatal message formats the wrong value

**Describe the problem**

The fatal in `StreamManager.NewStream` formats the `StreamManager` itself rather than the offending sender ([stream_manager.go#L110](https://github.com/cockroachdb/cockroach/blob/a7e117882b4356096b3a8ea26046baa7e5d53fce/pkg/kv/kvserver/rangefeed/stream_manager.go#L110)):

```go
log.KvExec.Fatalf(context.Background(), "unexpected sender type %T", sm)
```

Should be `sm.sender`. Unreachable with the two current sender implementations, but if a third sender is ever added, the crash message would report `*rangefeed.StreamManager` and mislead whoever debugs it.

**Additional context**

Found during an agent-assisted correctness audit of the rangefeed subsystem. Code link is pinned to master @ a7e1178.

Jira issue: CRDB-65658

Start in pkg/kv/kvserver/rangefeed/stream_manager.go at StreamManager.NewStream, specifically the fatal call around line 110. Verify which value the message formats and update the behavior so the reported type is the offending sender; done when the fatal message identifies that sender rather than the StreamManager.

Written by the indexing model from the issue text.