Function parse_native_tool_carrier 

pub fn parse_native_tool_carrier(content: &str) -> Option<CarrierParts>

Parse a native (role = "tool") carrier envelope.

Returns None when the content is not a JSON object with a string content field — those are raw-text tool results with no fixed position and are treated as legacy by callers. A present attachments key that is not an array is not a declaration: this runtime always writes an array, so a non-array value can only be a malformed envelope, and the conservative reading is legacy — zero attachments, untrusted body — agreeing with native_attachments. Array items that do not deserialize are skipped rather than failing the whole carrier.