Allow implicit Into for BSN enum field values

[bugsweeper]

Objective

• Allow BSN users to assign enum variants directly to fields when the enum implements Into for the field type.
• Fixes Allow BSN users to directly write enums as fields #24582.

Solution

• Route enum variant field values through the normal BsnValue token generation path.
• Make BsnValue::Type emit (...).into() when used as a value.
• Preserve existing enum component patching behavior, where entries like Foo::Bar { x: 1 } and Foo::Qux patch the enum component itself.

Testing

• cargo fmt --check -p bevy_scene -p bevy_scene_macros
• cargo test -p bevy_scene_macros enum_variant_field_values_use_implicit_into
• cargo test -p bevy_scene enum_variant_field_values_use_implicit_into
• cargo test -p bevy_scene enum_patching

Showcase

TextFont {
    font_size: TextSize::Large,
}