Add flag_missing error when requested flag is not in response

When the /flags API call succeeds but the requested flag is not in the
response, set $feature_flag_error to "flag_missing". This helps distinguish
between flags that don't exist vs other error conditions.

Author: haacked

posthog/client.py

@@ -1576,6 +1576,8 @@ def _get_feature_flag_result(
                 )
                 if errors_while_computing:
                     feature_flag_error = "errors_while_computing_flags"
+                elif flag_details is None:
+                    feature_flag_error = "flag_missing"
 
                 flag_result = FeatureFlagResult.from_flag_details(
                     flag_details, override_match_value

posthog/test/test_feature_flag_result.py

@@ -494,10 +494,9 @@ def test_get_feature_flag_result_with_errors_while_computing_flags(
     def test_get_feature_flag_result_flag_not_in_response(
         self, patch_capture, patch_flags
     ):
-        """Test that when a flag is not in the API response, we still capture the request_id.
+        """Test that when a flag is not in the API response, we capture flag_missing error.
 
-        This is the normal case when a flag doesn't exist or the user doesn't match
-        any conditions - the flag simply won't be in the response.
+        This happens when a flag doesn't exist or the user doesn't match any conditions.
         """
         patch_flags.return_value = {
             "flags": {
@@ -526,6 +525,7 @@ def test_get_feature_flag_result_flag_not_in_response(
                 "locally_evaluated": False,
                 "$feature/missing-flag": None,
                 "$feature_flag_request_id": "test-request-id-456",
+                "$feature_flag_error": "flag_missing",
             },
             groups={},
             disable_geoip=None,