feat(axum): parse typed interaction values

Add InteractionForm::parse for common typed field extraction and use it in example interaction handlers to avoid repeated string parsing plumbing.

req: form/004

req: dx/003

req: examples/003

slhx-axum/tests/response.rs

@@ -117,6 +117,17 @@ fn interaction_form_parses_handle_and_fields() {
     assert_eq!(form.values("tag").collect::<Vec<_>>(), ["a", "b/c"]);
 }
 
+#[test]
+fn interaction_form_parses_typed_values() {
+    // req: form/004 req: dx/003
+    let form = InteractionForm::parse_urlencoded(b"__h=42&count=7&bad=nope")
+        .expect("form should parse");
+
+    assert_eq!(form.parse::<u32>("count"), Some(7));
+    assert_eq!(form.parse::<u32>("bad"), None);
+    assert_eq!(form.parse::<u32>("missing"), None);
+}
+
 #[test]
 fn interaction_form_requires_numeric_handle() {
     assert_eq!(