C# generator erases referenced additionalProperties value schemas

## Description

The C# DirectClient generator erases a concrete model referenced as the value schema of `additionalProperties`.

Seismic Planner's reachable Swagger model has this shape:

```json
{
"CustomPropertyValues": {
"type": "object",
"properties": {
"localizations": {
"type": "object",
"additionalProperties": {
"$ref": "#/definitions/CustomPropertyDataDisplay"
}
}
}
},
"CustomPropertyDataDisplay": {
"type": "object",
"properties": {
"name": {
"type": "string"
}
}
}
}
```

At immutable .NET main commit [`70a1f5ed`](https://github.com/Azure/Connectors-NET-SDK/commit/70a1f5ed4a9f78980e32719622552be1dd7deb82), the generated client instead exposes [`JsonElement? Localizations`](https://github.com/Azure/Connectors-NET-SDK/blob/70a1f5ed4a9f78980e32719622552be1dd7deb82/src/Azure.Connectors.Sdk/Generated/SeismicPlannerExtensions.cs#L341), and `CustomPropertyDataDisplay` is not emitted anywhere in the file.

This was reproduced during [Python SDK PR 77](https://github.com/Azure/connectors-python-sdk/pull/77) validation by generating C# and Python from the same fresh ARM Swagger cache with [AzureUX-BPM generator PR 16929357 source commit `b57efae`](https://msazure.visualstudio.com/One/_git/AzureUX-BPM/commit/b57efae188a6971523cd822393721807dc2db637). The cross-language model-coverage check identified `CustomPropertyDataDisplay` as reachable from an operation contract but unmatched in generated C#.

The controlling fix belongs in the BPM CodefulSdkGenerator; this .NET SDK issue tracks the public generated-contract defect and required regeneration.

## Steps to reproduce

1. Export the Seismic Planner managed connector Swagger.
2. Generate its C# DirectClient with the CodefulSdkGenerator.
3. Inspect `CustomPropertyValues.Localizations` and the generated model declarations.
4. Observe that `Localizations` is `JsonElement?` and that no `CustomPropertyDataDisplay` class is generated, despite the reachable `additionalProperties.$ref`.

## Expected behavior

The generated contract should retain the map value schema, for example:

```csharp
[JsonPropertyName("localizations")]
public Dictionary Localizations { get; set; }

public class CustomPropertyDataDisplay
{
[JsonPropertyName("name")]
public string Name { get; set; }
}
```

The generator's reachable-model traversal should follow `$ref` values beneath `additionalProperties`, and its C# type mapper should preserve typed dictionary value shapes rather than degrading them to free-form JSON.

## SDK version

`main` at [`70a1f5ed`](https://github.com/Azure/Connectors-NET-SDK/commit/70a1f5ed4a9f78980e32719622552be1dd7deb82).

## .NET version

Generation defect; reproduced with .NET SDK `10.0.101` while building the generator's `net8.0` target.

## Additional context

This is distinct from:

- [Issue 157](https://github.com/Azure/Connectors-NET-SDK/issues/157), which intentionally types genuinely free-form JSON properties as `JsonElement?`. Here the source contract supplies a concrete value schema.
- [Issue 231](https://github.com/Azure/Connectors-NET-SDK/issues/231), which covers CLR member-name collisions, including reserving the synthetic `AdditionalProperties` member name.
- [Issue 232](https://github.com/Azure/Connectors-NET-SDK/issues/232), which was closed after separating unreachable definitions, typeless schemas, and Python wire-name collisions. This model is reachable through `CustomPropertyValues.properties.localizations.additionalProperties`.

Suggested acceptance criteria:

- Add a generator regression where a reachable property is an object whose `additionalProperties` value is a `$ref` to a concrete model.
- Emit `Dictionary` and retain `T` in the generated model graph.
- Cover nested/cyclic references without duplicate model emission or infinite traversal.
- Preserve existing free-form and primitive-map behavior.
- Regenerate Seismic Planner and add a JSON round-trip test with arbitrary locale keys and typed `CustomPropertyDataDisplay` values.
- Validate C# and Python model coverage from one immutable Swagger cache.