Function find_depth2_element_slice_by 

pub fn find_depth2_element_slice_by(
    body: &[u8],
    predicate: impl Fn(&str) -> bool,
) -> Option<&[u8]>

Locate a depth-2 XML element (a direct child of the document root) whose local name satisfies predicate, returning the byte slice covering it (<El>...</El>, inclusive of tags). The document root itself is also considered, so an “unwrapped” envelope whose root already matches is found.

Returns None if the body is not valid UTF-8, is not parseable as XML, or contains no matching element — callers decide how to fall back.

Robust to start-tag attributes (e.g. <Error xmlns="...">), nested same-name elements, comments, and CDATA, which a naive substring search would mishandle. Both the AWS REST XML error path (name == "Error") and the awsQuery response path (name.ends_with("Result") || name == "Error") build on this.