ProblemDetails.Extensions json tag is dead code (MarshalJSON never uses it) #19

`ProblemDetails.Extensions` is declared as:

```go
Extensions map[string]interface{} `json:"extensions,omitempty"`
```

but the type's custom `MarshalJSON` never serializes a nested `"extensions"` key — it flattens each entry to the top level instead (correctly, per RFC 9457). The struct tag is misleading: anyone reading the type definition without also reading `MarshalJSON` would reasonably expect a nested `extensions` object in the output, and it never appears.

Minor, but worth a fix — either drop the tag (since `MarshalJSON` bypasses it entirely) or replace it with a comment noting it's vestigial.