fix: switch to openai llm for e2e test since currently anthrpoic api has no balance

robot/e2e/m6_acceptance.robot

@@ -26,11 +26,11 @@ M6 Suite Setup
     Set Suite Variable    ${RUN_SUFFIX}    ${suffix}
     # Register the local/code-review action needed by plan lifecycle tests.
     # Pick an actor that matches the available API key.
-    ${has_anthropic}=    Evaluate    bool(__import__('os').environ.get('ANTHROPIC_API_KEY', ''))
-    IF    ${has_anthropic}
-        ${actor}=    Set Variable    anthropic/claude-sonnet-4-20250514
-    ELSE
+    ${has_openai}=    Evaluate    bool(__import__('os').environ.get('OPENAI_API_KEY', ''))
+    IF    ${has_openai}
         ${actor}=    Set Variable    openai/gpt-4o
+    ELSE
+        ${actor}=    Set Variable    anthropic/claude-sonnet-4-20250514
     END
     ${action_yaml}=    Catenate    SEPARATOR=\n
     ...    name: local/code-review