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>

tests/providers/test_provider_profiles.py

@@ -98,6 +98,11 @@ class TestOpenRouterProfile:
         body = p.build_extra_body(provider_preferences={"allow": ["anthropic"]})
         assert body["provider"] == {"allow": ["anthropic"]}
 
+    def test_extra_body_session_id(self):
+        p = get_provider_profile("openrouter")
+        body = p.build_extra_body(session_id="test-session-123")
+        assert body["session_id"] == "test-session-123"
+
     def test_extra_body_no_prefs(self):
         p = get_provider_profile("openrouter")
         body = p.build_extra_body()