Fix Anthropic SSE parsing for data: lines without spaces

[Hcaepllams]

Summary

• fix SSE parsing to accept data: lines without a trailing space
• restore Responses streaming when using Anthropic-compatible upstreams such as Kimi
• add a regression test covering the no-space data: format
• document the recommended Kimi -> Codex setup in the README

Problem

Some Anthropic-compatible upstreams emit SSE lines like:

data:{"type":"message_start", ...}                                                                                                                                                                         
                                                                                                                                                                                                           
instead of:                                                                                                                                                                                                
                                                                                                                                                                                                           
data: {"type":"message_start", ...}                                                                                                                                                                        
                                                                                                                                                                                                           
The parser only accepted data:  with a trailing space, so those events were dropped.                                                                                                                       
                                                                                                                                                                                                           
In practice this caused translated OpenAI Responses streams to appear empty, and Codex failed with:                                                                                                        
                                                                                                                                                                                                           
- stream closed before response.completed                                                                                                                                                                  
                                                                                                                                                                                                           
Fix                                                                                                                                                                                                        
                                                                                                                                                                                                           
take_one_sse_event() now accepts any data: prefix and trims the remainder before parsing JSON.                                                                                                             
                                                                                                                                                                                                           
Validation                                                                                                                                                                                                 
                                                                                                                                                                                                           
- added a unit test for data:{...} SSE lines                                                                                                                                                               
- verified responses_endpoint_streaming_translates_to_anthropic_upstream still passes                                                                                                                      
- verified a real Codex -> proxy -> Kimi flow now completes successfully