Unresolvable FEEL expression resolving as `null` with `Null` type

### Describe the Bug

As reported by @nikku in https://github.com/bpmn-io/variable-outline/pull/24#issuecomment-4003725371, we are displaying some variables' resolved values and types wrong.

With the following ioMapping declaration, I think we should have resolved `agentContext` as `Any` with `=agent.context` rather than `null` with `Null` type.

```xml

```

The variables are resolved as below from `variable-resolver`.

```
[
{
"name": "agentContext",
"type": "Null",
"info": "",
"entries": [],
"detail": "Null"
},
{
"name": "agent",
"provider": [
null
],
"type": "Any",
"info": "=agent",
"entries": [ ],
"detail": "Any"
}
]
```

### Steps to Reproduce

1. Open [this diagram](https://github.com/camunda/camunda-8-tutorials/blob/main/solutions/ai-agent-chat-with-tools/ai-agent-chat-with-tools.bpmn)
2. Focus on AI Agent ad-hoc subprocess
3. Inspect `agent` and `agentContext` variables in the new variables panel or `VariableResolver#getVariables`
4. While `agent` is `=agent`, `agentContext` is `null`

### Expected Behavior

1. `agentContext` should have been typed as `Any` with `=agent.context` value as it cannot be resolved

### Environment

- Library version: 2.0.0