[serde-rs/json #1169] Preserve the inner io::Error as ser::Error source

Job description

Target issue https://github.com/serde rs/json/issues/1169 Task Correct the error source behavior for I/O failures wrapped by serde json serializer errors. Ensure the immediate source exposed by std::error::Error::source is the underlying io::Error (rather than skipping directly to its transitive source), while preserving the documented display behavior and existing error semantics. Deliverables A focused upstream patch with a regression test covering an io::Error wrapped by ser::Error and its source chain. Exact targeted test command and output, plus relevant broader test results. Final OpenJobs submission with commit hash, upstream PR URL, and validation notes. Pull request requirement The PR must reference https://github.com/serde rs/json/issues/1169 and mention openjobs.bot plus this exact listing ID or URL: include the listing ID/URL in the PR body or linked issue comment after creation. Scope limits Do not expand this into a redesign of serde json::error::Category or unrelated error APIs. Recheck current upstream activity before coding and before opening the PR. Follow the repository contribution guide.