fix(cli): fix session list --format json empty-case structure (#6316)

Ensure the empty JSON path reuses the session summary builder so the machine-readable output aligns with the specification for empty data sets.

ISSUES CLOSED: #6316

Tests: nox -s unit_tests -- features/session_list_error.feature

features/session_list_error.feature

@@ -71,6 +71,7 @@ Feature: Session list command handles missing database gracefully
     Then the session-list-error command should exit successfully
     And the session-list-error output should be valid JSON containing "sessions"
     And the session-list-error output should not contain "AttributeError"
+    And the session-list-error JSON summary should represent an empty result
 
   @tdd_issue @tdd_issue_554
 #   @tdd_issue @tdd_issue_4270 @tdd_expected_fail @skip

features/steps/session_list_error_steps.py

@@ -251,6 +251,50 @@ def step_output_json_key(context: Context, key: str) -> None:
     )
 
 
+@then("the session-list-error JSON summary should represent an empty result")
+def step_json_empty_summary(context: Context) -> None:
+    result = context.sle_result
+    assert result is not None, "No command was invoked"
+    try:
+        parsed = json.loads(result.output)
+    except json.JSONDecodeError as exc:
+        raise AssertionError(f"Output is not valid JSON:\n{result.output}") from exc
+
+    data = _unwrap_envelope(parsed)
+    assert isinstance(data, dict), f"Expected JSON object, got {type(data)}: {data}"
+
+    sessions = data.get("sessions")
+    assert isinstance(sessions, list), (
+        f"Expected 'sessions' to be a list, got {type(sessions)}: {sessions}"
+    )
+    assert not sessions, f"Expected no sessions but found: {sessions}"
+
+    assert "total" not in data, (
+        "Top-level 'total' key should not be present; expected value nested in 'summary'."
+    )
+
+    summary = data.get("summary")
+    assert isinstance(summary, dict), (
+        f"Expected 'summary' to be a dict, got {type(summary)}: {summary}"
+    )
+
+    expected_keys = {"total", "most_recent", "oldest", "total_messages", "storage"}
+    missing_keys = expected_keys - set(summary)
+    assert not missing_keys, f"Missing summary keys: {sorted(missing_keys)}"
+
+    assert summary["total"] == 0, f"Expected summary total 0, got {summary['total']}"
+    assert summary["total_messages"] == 0, (
+        f"Expected total_messages 0, got {summary['total_messages']}"
+    )
+    assert summary["most_recent"] is None, (
+        f"Expected most_recent None, got {summary['most_recent']}"
+    )
+    assert summary["oldest"] is None, f"Expected oldest None, got {summary['oldest']}"
+    assert summary["storage"] == "0 KB", (
+        f"Expected storage '0 KB', got {summary['storage']}"
+    )
+
+
 @then('the session-list-error output should be valid YAML containing "{key}"')
 def step_output_yaml_key(context: Context, key: str) -> None:
     result = context.sle_result

src/cleveragents/cli/commands/session.py

@@ -292,7 +292,7 @@ def list_sessions(
         # For machine-readable formats, always emit a structured empty list
         # so that callers parsing JSON/YAML receive valid output.
         if fmt not in (OutputFormat.RICH.value, OutputFormat.COLOR.value):
-            typer.echo(format_output({"sessions": [], "total": 0}, fmt))
+            typer.echo(format_output(_session_list_dict([]), fmt))
             return
         console.print("[yellow]No sessions found.[/yellow]")
         console.print("Create one with 'agents session create'")