* feat(nous): send top-level session_id for provider sticky routing
The Nous Portal profile only embedded the session id inside portal tags,
so Claude traffic through the portal had no sticky-routing key. Multi-turn
sessions could reroute between upstream endpoints (Anthropic/Vertex/
Bedrock), cold-writing a fresh prompt cache on every reroute since each
provider's cache is instance-local.
Mirror the OpenRouter profile: emit extra_body.session_id whenever the
agent has one, pinning every turn of a session to the same endpoint so
explicit cache_control breakpoints stay warm.
* test: expect top-level session_id in Nous max-iterations summary body
Sibling site of the profile change — the max-iterations summary path
builds its request through the same NousProfile.build_extra_body(), so
its exact-shape assertion now includes the sticky-routing session_id
when the agent has a session.
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