Fix timeline view failing to open notebooks with chat edit changes

[Copilot]

Description

Timeline entries for notebooks with Copilot Edits fail to open with SyntaxError: Unexpected token '#', "# Display "... is not valid JSON because the timeline stores notebooks in an internal snapshot format (JSON.stringify([transientOptions, notebookData])) while the ipynb serializer expects standard Jupyter JSON.

Changes

• Detection: Check notebookViewType in getContentAtStop to identify notebook files
• Conversion: Added _convertNotebookSnapshotToSerializedFormat method that:
  • Deserializes internal snapshot to NotebookData
  • Uses INotebookSerializer.notebookToData() to convert to Jupyter JSON bytes
  • Decodes bytes to UTF-8 string
  • Returns valid empty notebook on conversion failure

// Before: returns internal snapshot format
return replayed.exists ? replayed.content : undefined;

// After: converts notebooks to Jupyter JSON
if (replayed.notebookViewType) {
    return this._convertNotebookSnapshotToSerializedFormat(replayed.content, replayed.notebookViewType);
}
return replayed.content;

Original prompt

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This section details on the original issue you should resolve

<issue_title>Timeline - can not open notebook with edit changes</issue_title>
<issue_description>Testing #241775

1. Open any notebook
2. Use Copilot Edits to make some changes to the notebook
3. Go to Timeline view
4. Click on Edits entry in timeline view -> Editor can not open

Error in dev console

SyntaxError: Unexpected token '#', "# Display "... is not valid JSON
    at JSON.parse (<anonymous>)
    at deserializeNotebook (/Applications/Visual Studio Code - Insiders.app/Contents/Resources/app/extensions/ipynb/dist/ipynbMain.node.js:1:167914)
    at Sh.$dataToNotebook (file:///Applications/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/workbench/api/node/extensionHostProcess.js:171:8563)
    at Vy.S (file:///Applications/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/workbench/api/node/extensionHostProcess.js:29:116244)
    at Vy.Q (file:///Applications/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/workbench/api/node/extensionHostProcess.js:29:116024)
    at Vy.M (file:///Applications/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/workbench/api/node/extensionHostProcess.js:29:115075)
    at Vy.L (file:///Applications/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/workbench/api/node/extensionHostProcess.js:29:114351)
    at Ah.value (file:///Applications/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/workbench/api/node/extensionHostProcess.js:29:113015)
    at P.B (file:///Applications/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/workbench/api/node/extensionHostProcess.js:27:2375)
    at P.fire (file:///Applications/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/workbench/api/node/extensionHostProcess.js:27:2593)
    at Jn.fire (file:///Applications/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/workbench/api/node/extensionHostProcess.js:29:9459)
    at Ah.value (file:///Applications/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/workbench/api/node/extensionHostProcess.js:190:13324)
    at P.B (file:///Applications/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/workbench/api/node/extensionHostProcess.js:27:2375)
    at P.fire (file:///Applications/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/workbench/api/node/extensionHostProcess.js:27:2593)
    at Jn.fire (file:///Applications/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/workbench/api/node/extensionHostProcess.js:29:9459)
    at MessagePortMain.<anonymous> (file:///Applications/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/workbench/api/node/extensionHostProcess.js:190:11616)
    at MessagePortMain.emit (node:events:518:28)
    at Object.MessagePortMain._internalPort.emit (node:electron/js2c/utility_init:2:2949)
    at Object.callbackTrampoline (node:internal/async_hooks:130:17)

fyi @jrieken </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Timeline - can not open notebook with edit changes #241892

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