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/src/lib.rs

@@ -243,6 +243,13 @@ impl InteractionForm {
             .find_map(|(field, value)| (field == name).then_some(value.as_str()))
     }
 
+    pub fn parse<T>(&self, name: &str) -> Option<T>
+    where
+        T: std::str::FromStr,
+    {
+        self.value(name).and_then(|value| value.parse().ok())
+    }
+
     pub fn values<'a>(&'a self, name: &'a str) -> impl Iterator<Item = &'a str> + 'a {
         self.fields
             .iter()