feat(nous): send top-level session_id for provider sticky routing (#69253)

* 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.

plugins/model-providers/nous/__init__.py

@@ -14,6 +14,15 @@ class NousProfile(ProviderProfile):
         self, *, session_id: str | None = None, **context
     ) -> dict[str, Any]:
         body: dict[str, Any] = {"tags": nous_portal_tags(session_id=session_id)}
+        if session_id:
+            # Top-level session_id → provider sticky routing key. Pins every
+            # turn of a session to the same upstream endpoint so explicit
+            # Anthropic cache_control breakpoints stay warm instead of
+            # cold-writing a fresh cache on each reroute (Anthropic/Vertex/
+            # Bedrock caches are instance-local). Mirrors the OpenRouter
+            # profile; without it the portal falls back to hashing the opening
+            # messages, which breaks pinning whenever those shift.
+            body["session_id"] = session_id
         provider_preferences = context.get("provider_preferences")
         if provider_preferences:
             body["provider"] = provider_preferences