fix(chat): add the boarder for the file and line up the undo icon (aws#7068)

## Problem


## Solution

![image](https://github.com/user-attachments/assets/a7fde94a-153b-4ae4-aba0-fa9e062259f7)
Add the boarder and background

---

- Treat all work as PUBLIC. Private `feature/x` branches will not be
squash-merged at release time.
- Your code changes must meet the guidelines in
[CONTRIBUTING.md](https://github.com/aws/aws-toolkit-vscode/blob/master/CONTRIBUTING.md#guidelines).
- License: I confirm that my contribution is made under the terms of the
Apache 2.0 license.

Author: Randall-Jiang

package-lock.json

@@ -525,7 +525,7 @@
         "@aws-sdk/s3-request-presigner": "<3.731.0",
         "@aws-sdk/smithy-client": "<3.731.0",
         "@aws-sdk/util-arn-parser": "<3.731.0",
-        "@aws/mynah-ui": "^4.31.0-beta.3",
+        "@aws/mynah-ui": "^4.31.0-beta.5",
         "@gerhobbelt/gitignore-parser": "^0.2.0-9",
         "@iarna/toml": "^2.2.5",
         "@smithy/fetch-http-handler": "^5.0.1",

packages/core/package.json

@@ -816,7 +816,7 @@ export class Messenger {
                 {
                     id: 'reject-code-diff',
                     status: 'clear',
-                    icon: 'revert' as MynahIconsType,
+                    icon: 'undo' as MynahIconsType,
                     text: 'Undo',
                 },
             ]
@@ -860,7 +860,7 @@ export class Messenger {
             new ChatMessage(
                 {
                     message: message,
-                    messageType: 'answer-part',
+                    messageType: toolUse?.name === ToolType.FsWrite ? 'answer' : 'answer-part',
                     followUps: undefined,
                     followUpsHeader: undefined,
                     relatedSuggestions: undefined,