test: add CHAT_CLAUDE_AGENT_CROSS_USER_PROMPT_CACHE to _CONFIG_ENV_VARS

test_default_config_is_enabled uses _clean_config_env to ensure env
vars don't pollute the ChatConfig constructor test.  The new
claude_agent_cross_user_prompt_cache field reads from
CHAT_CLAUDE_AGENT_CROSS_USER_PROMPT_CACHE, but that var was missing from
the list — leaving the test non-deterministic if that env var is set in CI.

autogpt_platform/backend/backend/copilot/sdk/service_test.py

@@ -398,6 +398,7 @@ _CONFIG_ENV_VARS = (
     "OPENAI_BASE_URL",
     "CHAT_USE_CLAUDE_CODE_SUBSCRIPTION",
     "CHAT_USE_CLAUDE_AGENT_SDK",
+    "CHAT_CLAUDE_AGENT_CROSS_USER_PROMPT_CACHE",
 )