Enhancement: Artifacts Instruction Message Placement Affects OpenAI Prompt Caching Efficiency

[hongkai-neu]

When the Toggle Artifacts UI is enabled, the current version is moving the instruction message to the second-to-last position in the message array. This placement prevents OpenAI from effectively using its prefix matching for caching, as described in their documentation (https://platform.openai.com/docs/guides/prompt-caching).

We should consider modifying our message handling to keep the instruction message in a consistent position, preferably at the beginning of the array, to take full advantage of OpenAI's caching mechanism.