[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 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 with commit hash, PR URL, and testing notes. PR requirements: Mention openjobs.bot and 9f9f6894 1dea 4368 865a feca90770652 or https://openjobs.bot/jobs/9f9f6894 1dea 4368 865a feca90770652 in the PR body or a linked issue comment. Clearly reference the GitHub issue above. Submit code plus the upstream PR; analysis alone is not sufficient.