fix(runtime): add err_kind field to remaining StepResult constructors

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: strefethen

crates/arazzo-runtime/src/runtime_core/engine_http.rs

@@ -261,6 +261,7 @@ impl Engine {
                 success: true,
                 response: Some(Arc::new(fake)),
                 err: None,
+                err_kind: None,
             },
             outputs,
             dry_run_request: Some(req),
@@ -345,6 +346,7 @@ impl Engine {
                         success: false,
                         response: Some(Arc::clone(response)),
                         err: None,
+                        err_kind: None,
                     },
                     outputs: BTreeMap::new(),
                     dry_run_request: None,
@@ -385,6 +387,7 @@ impl Engine {
                 success: true,
                 response: Some(Arc::clone(response)),
                 err: None,
+                err_kind: None,
             },
             outputs,
             dry_run_request: None,