Root cause: intent.timestamp was a naive datetime (no tzinfo). isoformat()
produces '2026-06-04T14:26:55.098914' (26 chars). The JSON prefix
'{"timestamp":"' is 14 chars → closing quote lands at column 41. Rust's
chrono::DateTime<Utc> serde rejects naive timestamps and serde_json reports
the error as 'premature end of input at line 1 column 41'.
Fix: _utc_isoformat() attaches UTC tzinfo before isoformat(), producing
'2026-06-04T14:26:55.098914+00:00' which chrono accepts.
Previous null-byte fix (_to_rust_bytes) and dangling-pointer fix (local vars)
remain correct and address real separate failure modes.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
acffc783e6