A pasted image renders as a broken thumbnail when its attachment has no contentType

A pasted image in an agent session renders as a broken thumbnail. The bytes are fine and the model receives the image; only the workbench fails to display it.

### Cause

The attachment is persisted without a `contentType`. Taken from a real session database:

```json
{"type":"embeddedResource","label":"Pasted Image","displayKind":"image","data":"iVBORw0KGgo…"}
```

There is no `contentType` key anywhere in that database. The restore path reads it unguarded:

```ts
if (attachment.type === MessageAttachmentKind.EmbeddedResource) {
if (!attachment.contentType.startsWith('image/')) {
```

so restoring the attachment throws and it never renders. Evaluated in the running workbench against that exact record:

```
(s) => s.contentType.startsWith("image/")
→ TypeError: Cannot read properties of undefined (reading 'startsWith')
```

`MessageEmbeddedResourceAttachment.contentType` is declared as a required `string`, so nothing in the type system anticipated the value the wire actually carries.

### Scope

`contentType` is dereferenced without a guard in four places: `stateToProgressAdapter` (the one users see), `codexPromptResolver` (twice) and `copilotAgentSession`.

Every current producer defaults the field, so newly pasted images are unaffected. This is about attachments already on disk, which stay broken for the life of the session.

### Reproduction

Any session whose stored attachment lacks `contentType` renders that attachment as a broken image. I have not identified which producer wrote one without it, and I would rather report that honestly than guess: the record predates the build I could inspect.

*AI disclosure: this issue and the related code were written with the assistance of AI.*

### Public patches and patcher scripts

[Public patch catalog and patcher scripts](https://github.com/RyanEwen/vscode-patches/blob/main/CATALOG.md) · [Source patch index](https://github.com/RyanEwen/vscode-patches/blob/main/SOURCE-PATCHES.md). The [public collection](https://github.com/RyanEwen/vscode-patches) includes the maintained patchers, rollback instructions, regression scripts, and historical snapshots. Build restrictions and exact installer coverage are documented there.