Fix index crash in agent starter (tldraw#7048)

This PR pulls across a fix from fairydraw into the agent starter.

### Change type

- [ ] `bugfix`
- [ ] `improvement`
- [ ] `feature`
- [ ] `api`
- [x] `other`

### Test plan

1. Create a shape...
2.

- [ ] Unit tests
- [ ] End to end tests

### Release notes

- Agent starter: Fixed a bug where shape indices could get set to the
same value.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Use the highest parent index for new shapes instead of a hardcoded
value across all shape converters to avoid index collisions.
> 
> -
**templates/agent/shared/format/convertSimpleShapeToTldrawShape.ts**:
> - **Index assignment**: When a shape's `index` is absent, use
`editor.getHighestIndexForParent(editor.getCurrentPageId())` instead of
the hardcoded `'a1'` for:
> - `text`, `line`, `arrow`, `geo`, `note`, `draw`, and `unknown`
shapes.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
d31e468. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: TodePond

templates/agent/shared/format/convertSimpleShapeToTldrawShape.ts

@@ -198,7 +198,7 @@ function convertTextShapeToTldrawShape(
 			x: correctedTextCoords.x,
 			y: correctedTextCoords.y,
 			rotation: defaultTextShape.rotation ?? 0,
-			index: defaultTextShape.index ?? ('a1' as IndexKey),
+			index: defaultTextShape.index ?? editor.getHighestIndexForParent(editor.getCurrentPageId()),
 			parentId: defaultTextShape.parentId ?? editor.getCurrentPageId(),
 			isLocked: defaultTextShape.isLocked ?? false,
 			opacity: defaultTextShape.opacity ?? 1,
@@ -242,7 +242,7 @@ function convertLineShapeToTldrawShape(
 			x: minX,
 			y: minY,
 			rotation: defaultLineShape.rotation ?? 0,
-			index: defaultLineShape.index ?? ('a1' as IndexKey),
+			index: defaultLineShape.index ?? editor.getHighestIndexForParent(editor.getCurrentPageId()),
 			parentId: defaultLineShape.parentId ?? editor.getCurrentPageId(),
 			isLocked: defaultLineShape.isLocked ?? false,
 			opacity: defaultLineShape.opacity ?? 1,
@@ -306,7 +306,7 @@ function convertArrowShapeToTldrawShape(
 		x: minX,
 		y: minY,
 		rotation: defaultArrowShape.rotation ?? 0,
-		index: defaultArrowShape.index ?? ('a1' as IndexKey),
+		index: defaultArrowShape.index ?? editor.getHighestIndexForParent(editor.getCurrentPageId()),
 		parentId: defaultArrowShape.parentId ?? editor.getCurrentPageId(),
 		isLocked: defaultArrowShape.isLocked ?? false,
 		opacity: defaultArrowShape.opacity ?? 1,
@@ -423,7 +423,7 @@ function convertGeoShapeToTldrawShape(
 			x: simpleShape.x ?? defaultGeoShape.x ?? 0,
 			y: simpleShape.y ?? defaultGeoShape.y ?? 0,
 			rotation: defaultGeoShape.rotation ?? 0,
-			index: defaultGeoShape.index ?? ('a1' as IndexKey),
+			index: defaultGeoShape.index ?? editor.getHighestIndexForParent(editor.getCurrentPageId()),
 			parentId: defaultGeoShape.parentId ?? editor.getCurrentPageId(),
 			isLocked: defaultGeoShape.isLocked ?? false,
 			opacity: defaultGeoShape.opacity ?? 1,
@@ -478,7 +478,7 @@ function convertNoteShapeToTldrawShape(
 			x: simpleShape.x ?? defaultNoteShape.x ?? 0,
 			y: simpleShape.y ?? defaultNoteShape.y ?? 0,
 			rotation: defaultNoteShape.rotation ?? 0,
-			index: defaultNoteShape.index ?? ('a1' as IndexKey),
+			index: defaultNoteShape.index ?? editor.getHighestIndexForParent(editor.getCurrentPageId()),
 			parentId: defaultNoteShape.parentId ?? editor.getCurrentPageId(),
 			isLocked: defaultNoteShape.isLocked ?? false,
 			opacity: defaultNoteShape.opacity ?? 1,
@@ -528,7 +528,7 @@ function convertDrawShapeToTldrawShape(
 			x: defaultDrawShape.x ?? 0,
 			y: defaultDrawShape.y ?? 0,
 			rotation: defaultDrawShape.rotation ?? 0,
-			index: defaultDrawShape.index ?? ('a1' as IndexKey),
+			index: defaultDrawShape.index ?? editor.getHighestIndexForParent(editor.getCurrentPageId()),
 			parentId: defaultDrawShape.parentId ?? editor.getCurrentPageId(),
 			isLocked: defaultDrawShape.isLocked ?? false,
 			opacity: defaultDrawShape.opacity ?? 1,
@@ -558,7 +558,7 @@ function convertUnknownShapeToTldrawShape(
 			x: simpleShape.x ?? defaultShape.x ?? 0,
 			y: simpleShape.y ?? defaultShape.y ?? 0,
 			rotation: defaultShape.rotation ?? 0,
-			index: defaultShape.index ?? ('a1' as IndexKey),
+			index: defaultShape.index ?? editor.getHighestIndexForParent(editor.getCurrentPageId()),
 			parentId: defaultShape.parentId ?? editor.getCurrentPageId(),
 			isLocked: defaultShape.isLocked ?? false,
 			opacity: defaultShape.opacity ?? 1,