feat(axum): build forms from checked handles

Add InteractionForm::for_handle so tests and public integration code can construct dispatched forms from generated Handle<T> values instead of raw numeric handle ids.

req: ceremony/004

req: public_api/001

slhx-axum/src/lib.rs

@@ -159,6 +159,10 @@ impl InteractionForm {
         }
     }
 
+    pub fn for_handle<I>(handle: Handle<I>, fields: impl IntoIterator<Item = (String, String)>) -> Self {
+        Self::new(handle.id().id, fields)
+    }
+
     pub fn parse_urlencoded(body: &[u8]) -> Result<Self, InteractionFormRejection> {
         Self::from_parts(parse_urlencoded_pairs(body)?, Vec::new())
     }