The image is cut from a request to a custom model

**Describe the bug**

Continue https://github.com/aaif-goose/goose/issues/10311 and https://github.com/aaif-goose/goose/pull/11496

Look to the screenshot. Sometime i got `[image omitted: model does not support vision]`

---

**To Reproduce**
Steps to reproduce the behavior:
0. Install 1.48.0
1. Go to 'New chat'
2. Drag and drop image
3. Chat `what is it?`
4. See thinking block

---

**Expected behavior**
The model returns the contents of the image

---

**Screenshots**

---

**Please provide the following information**
- **OS & Arch:** Windows 10
- **Interface:** UI
- **Version:** 1.48.0
- **Extensions enabled:** Computer Controller
- **Provider & Model:** Custom vLLM server with multimodal llm qwen3.8-27b

---

**Additional context**
Full provider config

```json
{
"name": "custom_qwen3_8-27b",
"engine": "openai",
"display_name": "qwen3.8-27b",
"description": "Custom qwen3.8-27b provider",
"api_key_env": "",
"base_url": "http://*:8000",
"models": [
{
"name": "qwen3.8-27b",
"context_limit": 524288, // change from default
"input_token_cost": null,
"output_token_cost": null,
"currency": null,
"supports_cache_control": null,
"reasoning": false,
"supports_vision": true // add to default
}
],
"headers": null,
"timeout_seconds": null,
"supports_streaming": true,
"requires_auth": false,
"catalog_provider_id": null,
"base_path": null,
"env_vars": null,
"dynamic_models": null,
"skip_canonical_filtering": true, // change from default
"model_doc_link": null,
"setup_steps": [],
"fast_model": null,
"preserves_thinking": true,
"emit_clear_thinking": false,
"setup": null
}
```

curl request is just working
```
{
"model": "qwen3.8-27b",
"messages": [
{
"role": "user",
"content": [
{
"type": "image_url",
"image_url": {
"url": "https://qianwen-res.oss-cn-beijing.aliyuncs.com/Qwen-VL/assets/demo.jpeg"
}
},
{"type": "text", "text": "what is it?"}
]
}
]
}
```
and return correct answer
```json
{
"role": "assistant",
"content": "\n\nIt's a photo of a woman and her dog on a sandy beach at sunset (or sunrise), with the warm, low sun flaring in from the right.\n\nThe main moment in the picture is the interaction between them:\n\n- **The dog** is a yellow ...",
"refusal": null,
"annotations": null,
"audio": null,
"function_call": null,
"reasoning": "The user's query is just \"what is it?\" which is ambiguous. They want me to identify the image content. Let me look carefully.\n\nThe ..."
}
```

The issue is about vision support for a custom model. Look at the provider configuration and the linked PR #11496 to understand how vision support is determined. Check the code that handles image attachments and sends requests to the OpenAI-compatible endpoint. The bug likely involves the model's `supports_vision` flag or how the image data is formatted in the request. Start by examining the provider engine logic and the request builder for multimodal messages.

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