feat(import): resolve references and schema composition

crates/crank-adapter-rest/src/error.rs

@@ -1,6 +1,26 @@
 use serde_json::Value;
 use thiserror::Error;
 
+#[derive(Debug, Error)]
+pub enum ExternalReferenceFetchError {
+    #[error("external references are disabled")]
+    Disabled,
+    #[error("external reference URL is invalid")]
+    InvalidUrl,
+    #[error("external reference target is not allowed")]
+    TargetNotAllowed,
+    #[error("external reference redirects are not allowed")]
+    RedirectNotAllowed,
+    #[error("external reference response exceeds the configured limit of {limit_bytes} bytes")]
+    ResponseTooLarge { limit_bytes: usize },
+    #[error("external reference endpoint returned status {status}")]
+    UnexpectedStatus { status: u16 },
+    #[error("external reference request failed")]
+    Transport { timeout: bool, connect: bool },
+    #[error("external reference fetch configuration is invalid")]
+    InvalidConfiguration,
+}
+
 #[derive(Debug, Error)]
 pub enum RestAdapterError {
     #[error("invalid base url: {url}")]