**Before:**
```
Error loading configuration: value `Never` is not in the allowed set [OnRequest]
```
**After:**
```
Error loading configuration: invalid value for `approval_policy`: `Never` is not in the
allowed set [OnRequest] (set by MDM com.openai.codex:requirements_toml_base64)
```
Done by introducing a new struct `ConfigRequirementsWithSources` onto
which we `merge_unset_fields` now. Also introduces a pair of requirement
value and its `RequirementSource` (inspired by `ConfigLayerSource`):
```rust
pub struct Sourced<T> {
pub value: T,
pub source: RequirementSource,
}
```
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