feat(openrouter): pass session_id in extra_body for sticky routing

OpenRouter supports a session_id field in extra_body that pins
multi-turn conversations to the same provider endpoint, enabling
prompt cache reuse across turns. The session_id was already threaded
through to build_extra_body() but never included in the returned dict.

Co-Authored-By: Claude Opus 4 (1M context) <noreply@anthropic.com>

plugins/model-providers/openrouter/__init__.py

@@ -43,6 +43,8 @@ class OpenRouterProfile(ProviderProfile):
         self, *, session_id: str | None = None, **context: Any
     ) -> dict[str, Any]:
         body: dict[str, Any] = {}
+        if session_id:
+            body["session_id"] = session_id
         prefs = context.get("provider_preferences")
         if prefs:
             body["provider"] = prefs