Fix markdown blocks being saved as type=code in .deepnote files (#176)

- Modified createBlockFromPocket to infer block type from cell kind when pocket metadata is missing
- Markdown cells (NotebookCellKind.Markup) now correctly default to type='markdown' instead of type='code'
- Added unit tests to verify markdown type preservation
- Ensures round-trip fidelity for markdown blocks even when __deepnotePocket metadata is not preserved

Author: jankuca

src/notebooks/deepnote/deepnoteDataConverter.unit.test.ts

@@ -176,6 +176,21 @@ suite('DeepnoteDataConverter', () => {
             assert.strictEqual(blocks[0].content, '## Heading');
         });
 
+        test('converts markdown cell without pocket to markdown block (not code)', () => {
+            const cells: NotebookCellData[] = [
+                {
+                    kind: NotebookCellKind.Markup,
+                    value: '# Title\n\nSome content',
+                    languageId: 'markdown'
+                }
+            ];
+
+            const blocks = converter.convertCellsToBlocks(cells);
+
+            assert.strictEqual(blocks[0].type, 'markdown');
+            assert.strictEqual(blocks[0].content, '# Title\n\nSome content');
+        });
+
         test('generates new IDs and sorting keys for cells without metadata', () => {
             const cells: NotebookCellData[] = [
                 {

src/platform/deepnote/pocket.ts

@@ -1,4 +1,4 @@
-import type { NotebookCellData } from 'vscode';
+import { NotebookCellKind, type NotebookCellData } from 'vscode';
 
 import type { DeepnoteBlock } from './deepnoteTypes';
 import { generateBlockId, generateSortingKey } from '../../notebooks/deepnote/dataConversionUtils';
@@ -64,13 +64,18 @@ export function createBlockFromPocket(cell: NotebookCellData, index: number): De
         }
     }
 
+    // Determine the block type:
+    // 1. Use the type from the pocket if available
+    // 2. Otherwise, infer from the cell kind (Code -> 'code', Markup -> 'markdown')
+    const defaultType = cell.kind === NotebookCellKind.Code ? 'code' : 'markdown';
+
     const block: DeepnoteBlock = {
         blockGroup: pocket?.blockGroup || generateUuid(),
         content: cell.value,
         id: cellId || generateBlockId(),
         metadata,
         sortingKey: pocket?.sortingKey || generateSortingKey(index),
-        type: pocket?.type || 'code'
+        type: pocket?.type || defaultType
     };
 
     if (pocket?.executionCount !== undefined) {

src/platform/deepnote/pocket.unit.test.ts

@@ -190,6 +190,31 @@ suite('Pocket', () => {
             assert.strictEqual(block.type, 'code');
         });
 
+        test('uses markdown type for markup cells when no pocket exists', () => {
+            const cell = new NotebookCellData(NotebookCellKind.Markup, '# Heading', 'markdown');
+
+            const block = createBlockFromPocket(cell, 0);
+
+            assert.strictEqual(block.type, 'markdown');
+        });
+
+        test('uses markdown type for markup cells when pocket has no type', () => {
+            const cell = new NotebookCellData(NotebookCellKind.Markup, '# Heading', 'markdown');
+
+            cell.metadata = {
+                __deepnotePocket: {
+                    sortingKey: 'a5'
+                },
+                id: 'markdown-block-123'
+            };
+
+            const block = createBlockFromPocket(cell, 0);
+
+            assert.strictEqual(block.type, 'markdown');
+            assert.strictEqual(block.id, 'markdown-block-123');
+            assert.strictEqual(block.sortingKey, 'a5');
+        });
+
         test('handles partial pocket data', () => {
             const cell = new NotebookCellData(NotebookCellKind.Code, 'print("hello")', 'python');