The web renderers handle `Image` inconsistently with each other and with the spec

The three web renderers each implement the basic catalog `Image` differently, leading to visibly different rendered output for the same A2UI input. They also diverge from the spec on the default `fit` value.

| Renderer | Implementation | Default `fit` |
|----------|----------------|---------------|
| React | Bare `` ([Image.tsx#L53](https://github.com/google/A2UI/blob/c55a37dc783146a607003ff3a3ecda92fbc36ac1/renderers/react/src/v0_9/catalog/basic/components/Image.tsx#L53)) | `'fill'` ([Image.tsx#L26](https://github.com/google/A2UI/blob/c55a37dc783146a607003ff3a3ecda92fbc36ac1/renderers/react/src/v0_9/catalog/basic/components/Image.tsx#L26)) — matches spec |
| Lit | `` inside an `a2ui-image` custom element ([Image.ts#L25](https://github.com/google/A2UI/blob/c55a37dc783146a607003ff3a3ecda92fbc36ac1/renderers/lit/src/v0_9/catalogs/basic/components/Image.ts#L25), [Image.ts#L87-L93](https://github.com/google/A2UI/blob/c55a37dc783146a607003ff3a3ecda92fbc36ac1/renderers/lit/src/v0_9/catalogs/basic/components/Image.ts#L87-L93)) with `width: 100%; height: auto` in shadow CSS ([Image.ts#L38-L44](https://github.com/google/A2UI/blob/c55a37dc783146a607003ff3a3ecda92fbc36ac1/renderers/lit/src/v0_9/catalogs/basic/components/Image.ts#L38-L44)) | `'fill'` inline ([Image.ts#L84](https://github.com/google/A2UI/blob/c55a37dc783146a607003ff3a3ecda92fbc36ac1/renderers/lit/src/v0_9/catalogs/basic/components/Image.ts#L84)), but the wrapper's `height: auto` might make it visually irrelevant? |
| Angular | Bare `` ([image.component.ts#L37-L44](https://github.com/google/A2UI/blob/c55a37dc783146a607003ff3a3ecda92fbc36ac1/renderers/angular/src/v0_9/catalog/basic/image.component.ts#L37-L44)) with `max-width: 100%; height: auto` in component CSS ([image.component.ts#L47-L52](https://github.com/google/A2UI/blob/c55a37dc783146a607003ff3a3ecda92fbc36ac1/renderers/angular/src/v0_9/catalog/basic/image.component.ts#L47-L52)) | `'cover'` ([image.component.ts#L78](https://github.com/google/A2UI/blob/c55a37dc783146a607003ff3a3ecda92fbc36ac1/renderers/angular/src/v0_9/catalog/basic/image.component.ts#L78)) — diverges from spec |
| Spec | | `'fill'` ([basic_catalog.json#L62-L67](https://github.com/google/A2UI/blob/c55a37dc783146a607003ff3a3ecda92fbc36ac1/specification/v0_9/json/basic_catalog.json#L62-L67)) |

One consequence of this can be seen in https://github.com/google/A2UI/issues/1297, which is an issue that the React angular has that the Lit and Angular ones don't.

Ideally, we want rendering behavior to be consistent and predictable across renderers, especially ones residing in the same repo and using the same underlying rendering tech (CSS). This is a larger problem that would require a great deal more work to fix and prevent regression for, so I leave it beyond the scope of this issue and focus on `Image` here instead.

We should pick a target behavior for `Image`, document it, and align all three renderers and the spec on it.