MCP image content blocks stripped from tool responses — AI cannot view images

### Description

When an MCP tool returns image content blocks (e.g., base64-encoded JPEG/PNG from `photoshop_get_preview`), the Copilot client strips or truncates the image data before passing the response to the AI model. The AI receives only the text/metadata portion and has no way to access the actual image.

### Expected Behavior

Image content blocks returned by MCP tools should be preserved and forwarded to the AI model, allowing it to "see" the image and provide visual-aware assistance.

### Actual Behavior

1. The MCP tool returns image data correctly (e.g., `photoshop_get_preview` returns `bytes: 30115`, indicating actual image content was generated)
2. The Copilot client strips the image content block from the response
3. The AI only receives text metadata: `{"ok": true, "width": 335, "height": 333, "bytes": 30115}`
4. The AI has no way to view the image unless the user manually takes a screenshot and sends it as a chat attachment

### Impact

- AI cannot verify visual editing results autonomously
- Cannot provide intelligent suggestions based on image content
- Requires manual screenshot sharing for every visual feedback loop, significantly reducing efficiency

### Reproduction Steps

1. Connect to a Photoshop MCP server, for example https://github.com/alisaitteke/photoshop-mcp
2. Open a document in Photoshop
3. Call `photoshop_get_preview`
4. Observe that the AI's response contains no image data

使用 Photoshop MCP server 和 photoshop_get_preview 工具复现该问题，然后追踪 MCP tool response 在到达 AI model 之前由 Copilot client 的哪个位置处理。完成标准是保留并转发 image content blocks，使 model 能够访问图像以及文本元数据。

由索引模型根据 Issue 内容生成。