The PR added helper-level tests for _trace_key but nothing exercised the
keys through the real hooks. This adds TestTurnTraceIsolation, which drives
on_pre_llm_request / on_post_llm_call across two turns of one gateway
session (task_id == session_id, unique turn_id, api_call_count reset per
turn) and asserts each turn opens its own root trace when the first turn
fails to finalize (tool-only final step). This test fails on the pre-fix
code (only one trace opened, turn 2 absorbed into turn 1) and passes with
the scoping fix.
Also pins the turn_id-over-api_request_id key precedence: the turn-scoped
post_llm_call carries no api_request_id, so it must still resolve to the
same key as the request-scoped hooks or finalization breaks.
b4356135f2