enable `reportUnreachable` in basedpyright

Keep it disabled for the tests in pyproject.toml
```
[[tool.basedpyright.executionEnvironments]]
root = "tests"
reportUnreachable  = false
```

 But you can remove this line from the more generic disable list.

reportUnreachable = false

The only violations are for INI integrations where we're raising an error - we can just remove this unnecessary error check.