fix: prevent struct errors from hitting plain-map normalize_error clause

[jscheid]

The normalize_error clause for %{message: message} matches any map with a :message key — including exception structs. When a struct hit this clause, Map.drop preserved __struct__ and __exception__ in the details, producing a map that masquerades as a struct but has the wrong fields. Downstream JSON encoding then crashes on the malformed map.

Add not is_struct(error) guard so structs dispatch to the dedicated struct clause, which uses Jido.Error.to_map for proper field extraction. Two tests that asserted :execution_error for InvalidInputError now correctly expect :validation_error.

[mikehostetler]

Leaving this open for now. We added the underlying transport-sanitizer foundation in agentjido/jido_action#134, and I think this jido_ai fix should follow that shared contract instead of broadening the local guard.

As written, the not is_struct(error) change fixes exception dispatch, but it still regresses ordinary non-exception structs with a :message field by sending them to the fallback path. Once the jido_action update is available here, we should revisit this on top of the new sanitizer/transport behavior and tighten the jido_ai side accordingly.