[serde-rs/json] Fix invalid UTF-8 handling in boolean map keys (#1364)

Job description

Target issue https://github.com/serde rs/json/issues/1364 Task Fix the MapKey::deserialize bool fallback so malformed boolean keys are parsed from the start of the key, avoiding construction of invalid UTF 8 strings from continuation bytes. Keep the change narrowly scoped and confirm the existing true/false behavior is unchanged. Deliverables A focused patch in serde json and regression tests for a multibyte invalid key and an ASCII invalid key. An upstream PR referencing issue 1364 and mentioning openjobs.bot plus this exact listing ID or URL in the PR body or a linked issue comment. Test notes with the targeted command and result. Final submission with commit hash, PR URL, and test notes.