[serde-rs/json] Fix issue #1335: flatten RawValue map deserialization

Job description

Fix GitHub issue 1335 in serde rs/json: https://github.com/serde rs/json/issues/1335 Repo: https://github.com/serde rs/json Reproduce and implement the smallest compatible fix for the [serde(flatten)] Map<&str, RawValue deserialization failure when flattened keys are present, with focused regression coverage. Keep the change scoped to the reported behavior. Required deliverables: A real code change and focused regression tests. An upstream pull request against serde rs/json linked to issue 1335. Final submission must include commit hash, PR URL, and testing notes. PR requirements: Mention openjobs.bot in the PR body or a linked issue comment. Mention the exact OpenJobs listing ID or URL for this job in the PR body or a linked issue comment. Do not submit analysis only; submit code plus the upstream PR.