Agent image icon fails to load (404): AppIcon receives the raw file id instead of icon_url

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### Dify version

1.16.1

### Cloud or Self Hosted

Self Hosted (Docker)

### Steps to reproduce

1. Enable Agent v2 (`ENABLE_AGENT_V2=true`) and create an Agent, e.g. `Test-agent`.
2. Set the Agent icon to an uploaded **image** (not an emoji). The upload itself succeeds: `POST /console/api/files/upload` returns 200 and a row is created in `upload_files`.
3. Reload the page and look at the Agent icon on the Agents list, or on the Agent configure page.

Requests observed in the browser devtools and in the nginx access log:

```
GET /29bdb007-4d8c-4888-83a2-7587abcafb26 404
GET /agents/019fd69e-7b20-79ee-8e15-b289e27928bc/29bdb007-4d8c-4888-83a2-7587abcafb26 404
```

The requested path is the **bare `upload_files.id`**. Because it is not an absolute path, the browser resolves it relative to the current page, which is why the same id produces a different (and always wrong) URL on `/agents` versus `/agents//configure`.

Agents whose icon is an **emoji** are not affected. Apps, datasets and the webapp site icon are not affected either — only Agent icons of type `image`.

### ✔️ Expected Behavior

The Agent icon renders using the signed preview URL that the backend already computes and returns as `icon_url`, e.g.

```
/files/29bdb007-4d8c-4888-83a2-7587abcafb26/file-preview?timestamp=...&nonce=...&sign=...
```

### ❌ Actual Behavior

A 404 and a broken image placeholder, everywhere an Agent icon of type `image` is rendered.

### Root cause

The backend is fine. `AgentAppDetailWithSite` and `AgentAppPartial` both inherit the `icon_url` computed field from `AppDetailWithSite`, and it resolves correctly. Verified in a 1.16.1 container:

```python
>>> AgentAppDetailWithSite.model_validate(app_model, from_attributes=True, ...).model_dump(mode="json")["icon_url"]
'/files/29bdb007-4d8c-4888-83a2-7587abcafb26/file-preview?timestamp=1786355545&nonce=...&sign=...'
```

The problem is on the web side. `AppIcon` uses the `imageUrl` prop directly as the `` source:

```tsx
// web/app/components/base/app-icon/index.tsx
112: const isValidImageIcon = iconType === 'image' && imageUrl
138:
```

But the Agent components pass `agent.icon` — the raw `upload_files` id — into `imageUrl`, where every other caller passes `icon_url`:

```tsx
// web/app/components/workflow/block-selector/agent-selector.tsx:230
imageUrl={agent.icon ?? undefined}

// web/app/components/workflow/nodes/agent-v2/components/agent-roster-field.tsx:73
imageUrl={agent.icon ?? undefined}
```

Compare with the correct pattern used elsewhere, e.g. app / dataset / site icons:

```tsx
imageUrl={app.icon_url}
```

The same `imageUrl: X.icon` pattern also appears in the shipped `dify-web:1.16.1` bundle in the Agent configure page header component (the chunk containing `i-custom-vender-agent-v2-configure`), so there is likely a third call site besides the two above. In the built output the bad pattern occurs only in Agent-related chunks:

```
$ grep -oE "imageUrl:[A-Za-z_$]+\.icon(\?\?|,|\}|\))" .js
# matches only in the agent header and block-selector chunks;
# all app/dataset/site chunks use imageUrl:X.icon_url
```

### Suggested fix

Pass `icon_url` instead of `icon` in the Agent call sites, falling back to `undefined`:

```diff
- imageUrl={agent.icon ?? undefined}
+ imageUrl={agent.icon_url ?? undefined}
```

`icon_url` is already present on both the Agent detail and the Agent list (roster) responses, so no backend change is needed.

Possibly related: #39447 (closed, reported against 1.16.0).

Start in web/app/components/workflow/block-selector/agent-selector.tsx and web/app/components/workflow/nodes/agent-v2/components/agent-roster-field.tsx, then locate the additional Agent configure-page call site in the built bundle or its source. Compare these uses with app, dataset, and site icon callers, and verify that every Agent image icon renders from icon_url and no longer requests the raw upload file id.

Written by the indexing model from the issue text.