TF_LOG doesn't act as a most-verbose clamp in the sink

### terraform-plugin-log version

```
main@5013651f4e1e1c1433800ff3172f6f4eaebdd66f
```

If you set `TF_LOG` to `ERROR` and a provider-defined environment variable, or even `TF_LOG_SDK` to `DEBUG`, the SDK logs will actually be logged at `DEBUG` verbosity. Intuitively, you'd imagine that they would _only_ be logged at `ERROR`, because `TF_LOG` should override other environment variables when it's less verbose.

... I think, right?

Start by reproducing the logging configuration with TF_LOG=ERROR alongside a provider-defined variable or TF_LOG_SDK=DEBUG. Trace how the sink combines these verbosity settings; done means TF_LOG acts as a maximum verbosity so SDK logs are not emitted above ERROR.

Written by the indexing model from the issue text.