.Net - Format agent output for AskAsync (agent as a plug-in) (#4907)

### Motivation and Context

<!-- Thank you for your contribution to the semantic-kernel repo!
Please help reviewers and future users, providing the following
information:
  1. Why is this change required?
  2. What problem does it solve?
  3. What scenario does it contribute to?
  4. If it fixes an open issue, please link to the issue here.
-->

Agent output unclear for multi-turn interactions:

```
Message1Message2Message3
```

### Description

<!-- Describe your changes, the overall approach, the underlying design.
These notes will help understanding how your code works. Thanks! -->

Separate message on a new line:

```
Message1
Message2
Message3
```
### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [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, and I have added new tests where possible
- [x] I didn't break anyone 😄

Co-authored-by: Gil LaHaye <[email protected]>
Co-authored-by: Teresa Hoang <[email protected]>

Author: 3 people

dotnet/src/Experimental/Agents/Internal/Agent.cs

@@ -245,7 +245,7 @@ private async Task<AgentResponse> AskAsync(
                 new AgentResponse
                 {
                     ThreadId = thread.Id,
-                    Message = string.Concat(messages.Select(m => m.Content)),
+                    Message = string.Join(Environment.NewLine, messages.Select(m => m.Content)),
                 };
 
             return response;