.Net: KernelProcessEventData deserialized in LocalAgentStep (#13203)

### Motivation and Context

1. **Why is this change required?**
Because `LocalAgentStep` was passing `KernelProcessEventData` wrappers
directly into `KernelArguments`, unlike `LocalStep` which unwraps them.
This inconsistency caused agents to receive JSON-like data instead of
the actual payload.

2. **What problem does it solve?**
It ensures that agents receive the real value (e.g. the user’s message
text) instead of the wrapper object, preventing confusing model
responses like *“you seem to be passing a json”*.

3. **What scenario does it contribute to?**
Agent-based process pipelines (for example a simple chat process where
user input is sent to an agent). The agent now correctly sees the user
input as plain text or object, improving reliability and user
experience.

### Description

This PR updates `LocalAgentStep` so that it unwraps
`KernelProcessEventData` before passing it into
`KernelArguments["message"]`.

* If `TargetEventData` is a `KernelProcessEventData`, we now call
`.ToObject()` to get the real value.
* If it is not, we keep the value as it is.
* This makes `LocalAgentStep` consistent with
`LocalStep.AssignStepFunctionParameterValues`.

With this change, agents no longer receive the wrapper object but the
actual payload (for example the plain user input), which avoids the
model interpreting it as JSON.

### Contribution Checklist

- [X] The code builds clean without any errors or warnings
- [X] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [X] All unit tests pass
- [X] I didn't break anyone 😄

Co-authored-by: Dmytro Struk <[email protected]>

Author: Laurianti

dotnet/src/Experimental/Process.LocalRuntime/LocalAgentStep.cs

@@ -45,7 +45,16 @@ internal override async Task HandleMessageAsync(ProcessMessage message)
         await this._initializeTask.Value.ConfigureAwait(false);
 
         string targetFunction = "Invoke";
-        KernelArguments arguments = new() { { "message", message.TargetEventData }, { "writtenToThread", message.writtenToThread == this._agentThread.ThreadId } };
+        KernelArguments arguments = new()
+        {
+            { "message", message.TargetEventData switch
+                {
+                    KernelProcessEventData proxyData => proxyData.ToObject(),
+                    _ => message.TargetEventData
+                }
+            },
+            { "writtenToThread", message.writtenToThread == this._agentThread.ThreadId }
+        };
         if (!this._functions.TryGetValue(targetFunction, out KernelFunction? function) || function == null)
         {
             throw new ArgumentException($"Function Invoke not found in plugin {this.Name}");