Replace adapter-based extraction in traced_llm with direct reads from
_settings.system_instruction (priority) and context messages (fallback).
The old approach had three bugs: signature mismatch with Anthropic
adapter, key name inconsistency, and unnecessary overhead from full
message/tools conversion.
Also deduplicate the system instruction in spans -- it was appearing as
both "system" and "param.system_instruction".
991fbb82da