feat(avatar): add styles for default (round) shape in ionic theme (#29550)

Issue number: internal

---------

## What is the current behavior?
Avatar does not have styles for the `"round"` shape in the ionic theme.

## What is the new behavior?
- Adds the styles for the default (round) shape (border radius)
- Adds e2e test for the round shape
- Updates the screenshots for the avatar `size` due to the new default
shape

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

## Other information

[Preview](https://ionic-framework-git-rou-10738-ionic1.vercel.app/src/components/avatar/test/shape?ionic:theme=ionic)

packages/vue/src/proxies.ts

@@ -103,7 +103,8 @@ export const IonAccordionGroup = /*@__PURE__*/ defineContainer<JSX.IonAccordionG
 
 
 export const IonAvatar = /*@__PURE__*/ defineContainer<JSX.IonAvatar>('ion-avatar', defineIonAvatar, [
-  'size'
+  'size',
+  'shape'
 ]);