不支持图片的模型在上下文压缩时因 image_url 内容报 400 错误

Title:

  [bug] Compaction fails with image_url error on non-vision models                                                                                                  

Body:

  ## Description                                                                                                                                                    
                                                                                                                                                                    
  When using a model that does not support images (e.g. some custom models without vision capabilities), the context compaction pipeline sends `image_url` type     
content in the messages, causing a 400 error from the API provider.                                                                                                 
                                                                                                                                                                    
  ## Steps to Reproduce                                                                                                                                             
                                                                                                                                                                    
  1. Configure a model that does not support image input                                                                                                            
  2. Have a conversation where images appear (auto-detected by tools)                                                                                               
  3. Trigger context compaction (or continue the conversation)                                                                                                      
                                                                                                                                                                    
  ## Error                                                                                                                                                          
                                                                                                                                                                    

[compaction.failed] APIStatusError: 400 Error from provider:
Upstream request failed: [invalid_request_error] Failed to deserialize the JSON body into the target type: messages[124]: unknown variant image_url, expected text
at line 1 column 811568

                                                                                                                                                                    
  ## Expected Behavior                                                                                                                                              
                                                                                                                                                                    
  Before sending API requests, Kimi Code should detect whether the target model supports image input. If not, `image_url` type content should be filtered out or    
converted to plain text descriptions.                                                                                                                               
                                                                                                                                                                    
  ## Environment                                                                                                                                                    
                                                                                                                                                                    
  - Kimi Code CLI: latest version                                                                                                                                   
  - Model: custom non-vision model                                                                                                                                  
  - OS: Windows 

Start by tracing the context compaction pipeline and the API request message construction. Reproduce the failure with a custom non-vision model and a conversation containing images, then verify that requests no longer send unsupported image_url content and that compaction succeeds.