Fix ClassDecoratorSetting and get_class_settings to correctly handle __databind_settings__ on subclasses

[Copilot]

ClassDecoratorSetting.__call__ used getattr() to look up __databind_settings__, which resolves through MRO and returns the parent's list when the subclass has none. It then appends to that parent list in-place — but get_class_settings() originally used vars(), which only checks the class's own __dict__, so the setting was never found on the subclass. Net effect: @ExtraKeys() on a subclass whose parent was already decorated was silently a no-op.

Additionally, get_class_settings() did not traverse the MRO, meaning subclasses could not inherit settings (such as ExtraKeys()) from parent classes at all.

Changes

• settings.py — ClassDecoratorSetting.__call__: Replace getattr(type_, "__databind_settings__", None) with vars(type_).get("__databind_settings__", None), ensuring a new list is always created on the target class rather than reusing an inherited one.

• settings.py — get_class_settings: Updated to traverse type_.__mro__ so that subclasses inherit ClassDecoratorSetting settings (e.g. ExtraKeys()) from parent classes. The child class's own settings are yielded first, so a child can override a parent's setting by decorating itself explicitly (e.g. @ExtraKeys(allow=False)).

• converters_test.py: Add three regression tests covering the scenario:

  • Both parent and child decorated with @ExtraKeys() each get independent __databind_settings__ lists, and the parent's list is not polluted.
  • A decorated subclass correctly allows extra keys during deserialization.
  • An undecorated subclass inherits ExtraKeys() from its parent (extra keys allowed), while a child explicitly decorated with @ExtraKeys(allow=False) overrides that and rejects extra keys.

@ExtraKeys()
@dataclasses.dataclass
class Parent:
    a: int

@dataclasses.dataclass
class Child(Parent):  # Inherits ExtraKeys() from Parent via MRO
    b: str = ""

mapper.deserialize({"a": 1, "b": "hello", "extra": "ignored"}, Child)  # Now works correctly

@ExtraKeys(allow=False)
@dataclasses.dataclass
class ChildOverriding(Parent):  # Overrides parent's ExtraKeys()
    b: str = ""

mapper.deserialize({"a": 1, "b": "hello", "extra": "ignored"}, ChildOverriding)  # Raises ConversionError

Original prompt

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This section details on the original issue you should resolve

<issue_title>ExtraKeys decorator does not create own databind_settings on subclasses</issue_title>
<issue_description>When applying ExtraKeys() to a subclass that has no own __databind_settings__, the decorator's getattr() call falls back to the parent's list via MRO traversal and appends to it in place, rather than creating a new list on the subclass. As a result, get_class_settings(), which uses vars(), finds no setting on the subclass, making ExtraKeys effectively a no-op for it. The bug is order-dependent: it only triggers when ExtraKeys() is applied to a subclass whose parent was already decorated.</issue_description>

<agent_instructions>Include at least one at max three test cases that cover the described sccenario.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes ExtraKeys decorator does not create own __databind_settings__ on subclasses #76

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