[Bug]: http pd router only use the first message

[Moersity]

Bug Description

In the SGLang PD router path for OpenAI-compatible chat completions, the cache-aware routing text is currently built only from the first chat message:
see https://github.com/lightseekorg/smg/blob/main/model_gateway/src/routers/http/pd_router.rs#L1445

sglang router also has this problem but already fixed by sgl-project/sglang#27430

Steps to Reproduce

1. Config smg as http pd router.
2. Continuous chat multi turn
3. Cache hit rate is extremely low

Expected Behavior

routing with full conversation

Actual Behavior

only route with first message

Component

model-gateway (core routing)

Routing Policy (if applicable)

None

Connection Mode

None

Configuration

Logs / Error Output

Environment

os: linux

Deployment Environment

Docker

Streaming Context

• Issue occurs with streaming (SSE) responses
• Issue occurs with non-streaming responses
• Issue occurs in both modes

Additional Context

No response

Pre-submission Checklist

• I have searched existing issues and discussions
• I can reproduce this issue consistently
• I am using the latest version of SMG