Adds example-request-only-with-warnings, a metadata recipe that reports warnings through the new json_warnings path of its mapping config.
Unlike json_error, warnings do not fail the mapping — the values still apply and fylr passes the texts on to whoever triggered the run; a background task writes them to its log. Mirrors example-request-only-with-error for the other half of what a recipe can report, so fylr's apitest can cover that path with no licensed plugin and no external service (#80512).