Honor warning instance categories before validating category arguments (#9034)

Extract the Python 3.14-compatible ordering fix from #8954. An existing Warning instance determines its own category, regardless of the category argument. Keep canonical invalid-category coverage and add only the missing instance case to the existing warnings snippet.

Assisted-by: OpenAI Codex:model identifier unavailable

crates/vm/src/stdlib/_warnings.rs

@@ -100,15 +100,12 @@ mod _warnings {
         category: Option<PyObjectRef>,
         vm: &VirtualMachine,
     ) -> PyResult<Option<PyTypeRef>> {
+        if message.fast_isinstance(vm.ctx.exceptions.warning) {
+            return Ok(Some(message.class().to_owned()));
+        }
         let cat_obj = match category {
             Some(c) if !vm.is_none(&c) => c,
-            _ => {
-                return Ok(if message.fast_isinstance(vm.ctx.exceptions.warning) {
-                    Some(message.class().to_owned())
-                } else {
-                    None // will default to UserWarning in warn_explicit
-                });
-            }
+            _ => return Ok(None), // will default to UserWarning in warn_explicit
         };
 
         let cat = PyTypeRef::try_from_object(vm, cat_obj.clone()).map_err(|_| {

extra_tests/snippets/stdlib_warnings.py

@@ -1,3 +1,20 @@
 import _warnings
 
 _warnings.warn("Test")
+
+
+import warnings
+
+
+class CustomWarning(UserWarning):
+    pass
+
+
+for category in (None, DeprecationWarning, int, 42):
+    message = CustomWarning("sentinel")
+    with warnings.catch_warnings(record=True) as caught:
+        warnings.simplefilter("always")
+        warnings.warn(message, category)
+    assert len(caught) == 1
+    assert caught[0].message is message
+    assert caught[0].category is CustomWarning