…h correct value: 'grid'" (And add MessagingExtensionAttachmentLayout) (#382)

Reverts #381

This is a bit of mess - 
* There is an `AttachmentLayout` that exists on `Activity` which should
be `list | carousel`.
* There is an `AttachmentLayout` that exists on `MessageExtension` which
should be `list | grid`.

Aligns with <a
href="https://github.com/microsoft/teams.ts/pull/381">teams.ts PR
#381</a> to add "grid" layout support for messaging extensions while
maintaining backward compatibility.

## Changes

- **New Enum**: Created `MessagingExtensionAttachmentLayout` with values
`LIST = "list"` and `GRID = "grid"`
- **Updated Type**: `MessagingExtensionResult.attachment_layout` now
uses `MessagingExtensionAttachmentLayout` instead of `AttachmentLayout`
- **Preserved Original**: `AttachmentLayout` remains unchanged with
`LIST` and `CAROUSEL` for backward compatibility
- **Exports**: Added `MessagingExtensionAttachmentLayout` to
`messaging_extension/__init__.py`
- **Test Updates**: Updated test applications to use the new enum type

## Usage

```python
from microsoft.teams.api.models import (
    MessagingExtensionAttachmentLayout,
    MessagingExtensionResult
)

result = MessagingExtensionResult(
    attachment_layout=MessagingExtensionAttachmentLayout.GRID,  # New GRID option
    attachments=[...]
)

# AttachmentLayout.CAROUSEL still available for other use cases
from microsoft.teams.api.models import AttachmentLayout
# Existing code using AttachmentLayout.CAROUSEL continues to work
```

**Note**: This change maintains backward compatibility. Existing code
using `AttachmentLayout.CAROUSEL` is not affected. Only messaging
extension code needs to migrate to the new
`MessagingExtensionAttachmentLayout` type.

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<details>

<summary>Original prompt</summary>

> can you change AttachmentLayout to be list | grid instead of list |
carousel. Link <a href="microsoft/teams.ts#381"
rel="noreferrer noopener"
title="microsoft/teams.ts#381"
target="_blank">https://github.com/microsoft/teams.ts/pull/381</a> in
the PR description. <attachment
id="app-preview-card-psc7b00c7e591949669b47bbed5c303ae2"></attachment>


</details>



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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: heyitsaamir <[email protected]>