feat(api): add #[non_exhaustive] to public types likely to evolve

ForwardingAction, TargetPattern, ForwardingRule, OperationType,
InterfaceConfig, InterfaceKind, DynamicConfig, and CallError are all
likely to gain variants/fields in future phases. Marking them
#[non_exhaustive] now prevents downstream breakage when new
variants/fields are added. Added constructor methods for types that
are constructed from other crates.

crates/alknet-core/src/call/response.rs

@@ -2,12 +2,23 @@ use serde::{Deserialize, Serialize};
 use serde_json::Value;
 
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+#[non_exhaustive]
 pub struct CallError {
     pub code: String,
     pub message: String,
     pub retryable: bool,
 }
 
+impl CallError {
+    pub fn new(code: impl Into<String>, message: impl Into<String>, retryable: bool) -> Self {
+        Self {
+            code: code.into(),
+            message: message.into(),
+            retryable,
+        }
+    }
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct ResponseEnvelope {
     pub request_id: String,