Function canonical_string 

pub fn canonical_string(
    key_id: &str,
    method: &Method,
    path: &str,
    query: Option<&str>,
    timestamp: &str,
    nonce: &str,
    body_sha256_hex: &str,
) -> String

Build the canonical string (DW-036 v1). The elements are the PRESENTED header values and the request target EXACTLY as received — the signer’s bytes and the verifier’s bytes must agree without any normalization step (see the module docs for the grammar and the rationale). Public because the grammar is the interop contract: tests re-implement it independently to pin the documentation, and operators may build signing tooling against the same function the gateway verifies with.