feat(langchain): support SystemMessage in create_agent's system_prompt

[sydney-runkle]

• create_agent's system_prompt allows str | SystemMessage
• added system_message: SystemMessage on ModelRequest
• ModelRequest.system_prompt is a function of system_message.text, now deprecated
• disallow setting system_prompt and system_message
• ModelRequest.system_prompt can still be set (w/ custom setattr) for custom backwards compat, but the updates just get propogated to the ModelRequest.system_message

[codspeed-hq]

CodSpeed Performance Report

Merging #34055 will not alter performance

_{Comparing sr/system-prompt-again (bd4c53e) with master (eb0545a)}

Summary

✅ 3 untouched
⏩ 31 skipped¹

Footnotes

1. 31 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩