Add X-Session-Id and X-Session-Affinity headers to all outbound LLM chat
completion requests so LLM gateways can sticky-route consecutive requests
from the same conversation and observability tools (Langfuse, Arize) can
group stateless API calls into a single multi-turn trace.
The headers carry a stable identifier: the task id when available (stable
across pause/resume), otherwise the pi session id. The implementation wraps
modelRegistry.getApiKeyAndHeaders -- the single chokepoint pi-coding-agent
uses for both the main stream and compaction -- merging routing headers into
the resolved output. This covers all HTTP-based providers (built-in, custom,
and HTTP-streaming extensions) without disturbing auth resolution.
Also propagates taskId to four secondary executor sessions (retry,
verification-fix, workflow-step, child-agent) that previously fell back to
a per-instance pi id, fragmenting per-task observability grouping.
Closes#1675
e17e9bc867