feat(amazonq): Enable users to edit code files directly on the diff view (aws#8019)

## Problem
Currently users couldn't edit code files on the diff view provided by
Amazon Q. Users have to open those files in the workspace to edit them.
This is very inconvenient.

## Solution
Copy of aws#7886

Enable users to edit code files directly on the diff view.


https://github.com/user-attachments/assets/464d9757-cb6f-4f0f-aa6f-de4f3104cdb5

---

- 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: ashishrp-aws

packages/amazonq/src/lsp/chat/messages.ts

@@ -69,6 +69,7 @@ import {
 } from '@aws/language-server-runtimes/protocol'
 import { v4 as uuidv4 } from 'uuid'
 import * as vscode from 'vscode'
+import * as path from 'path'
 import { Disposable, LanguageClient, Position, TextDocumentIdentifier } from 'vscode-languageclient'
 import { AmazonQChatViewProvider } from './webviewProvider'
 import {
@@ -81,22 +82,8 @@ import {
     SecurityIssueTreeViewProvider,
     CodeWhispererConstants,
 } from 'aws-core-vscode/codewhisperer'
-import {
-    amazonQDiffScheme,
-    AmazonQPromptSettings,
-    messages,
-    openUrl,
-    isTextEditor,
-    globals,
-    setContext,
-} from 'aws-core-vscode/shared'
-import {
-    DefaultAmazonQAppInitContext,
-    messageDispatcher,
-    EditorContentController,
-    ViewDiffMessage,
-    referenceLogText,
-} from 'aws-core-vscode/amazonq'
+import { AmazonQPromptSettings, messages, openUrl, isTextEditor, globals, setContext } from 'aws-core-vscode/shared'
+import { DefaultAmazonQAppInitContext, messageDispatcher, referenceLogText } from 'aws-core-vscode/amazonq'
 import { telemetry } from 'aws-core-vscode/telemetry'
 import { isValidResponseError } from './error'
 import { decryptResponse, encryptRequest } from '../encryption'
@@ -664,31 +651,42 @@ export function registerMessageListeners(
     )
 
     languageClient.onNotification(openFileDiffNotificationType.method, async (params: OpenFileDiffParams) => {
-        const ecc = new EditorContentController()
-        const uri = params.originalFileUri
-        const doc = await vscode.workspace.openTextDocument(uri)
-        const entireDocumentSelection = new vscode.Selection(
-            new vscode.Position(0, 0),
-            new vscode.Position(doc.lineCount - 1, doc.lineAt(doc.lineCount - 1).text.length)
-        )
-        const viewDiffMessage: ViewDiffMessage = {
-            context: {
-                activeFileContext: {
-                    filePath: params.originalFileUri,
-                    fileText: params.originalFileContent ?? '',
-                    fileLanguage: undefined,
-                    matchPolicy: undefined,
-                },
-                focusAreaContext: {
-                    selectionInsideExtendedCodeBlock: entireDocumentSelection,
-                    codeBlock: '',
-                    extendedCodeBlock: '',
-                    names: undefined,
-                },
-            },
-            code: params.fileContent ?? '',
+        const currentFileUri = vscode.Uri.parse(params.originalFileUri)
+        const originalContent = params.originalFileContent ?? ''
+        const fileName = path.basename(currentFileUri.fsPath)
+
+        // Use custom scheme to avoid adding to recent files
+        const originalFileUri = vscode.Uri.parse(`amazonq-diff:${fileName}_original_${Date.now()}`)
+
+        // Register content provider for the custom scheme
+        const disposable = vscode.workspace.registerTextDocumentContentProvider('amazonq-diff', {
+            provideTextDocumentContent: () => originalContent,
+        })
+
+        try {
+            // Open diff view with custom scheme URI (left) vs current file (right)
+            await vscode.commands.executeCommand(
+                'vscode.diff',
+                originalFileUri,
+                currentFileUri,
+                `${vscode.workspace.asRelativePath(currentFileUri)} (Original ↔ Current, Editable)`,
+                { preview: false }
+            )
+
+            // Clean up content provider when diff view is closed
+            const cleanupDisposable = vscode.window.onDidChangeVisibleTextEditors(() => {
+                const isDiffViewOpen = vscode.window.visibleTextEditors.some(
+                    (editor) => editor.document.uri.toString() === originalFileUri.toString()
+                )
+                if (!isDiffViewOpen) {
+                    disposable.dispose()
+                    cleanupDisposable.dispose()
+                }
+            })
+        } catch (error) {
+            disposable.dispose()
+            languageClient.error(`[VSCode Client] Failed to open diff view: ${error}`)
         }
-        await ecc.viewDiff(viewDiffMessage, amazonQDiffScheme)
     })
 
     languageClient.onNotification(chatUpdateNotificationType.method, (params: ChatUpdateParams) => {