Fix lifetime issues for native subexpressions in bsn! enum variants

[akshitj11]

Summary

bsn! lifts {...} expressions into let _exprN = ... before building SceneFunction closures so they can be 'static. That already worked for plain struct fields via process_field, but not for fields inside enum variant patches like Foo::Baz({expr}), which went through process_enum_field and left the borrow inline in the closure.

This adds the same hoisting branch to process_enum_field.

Fixes #24775

Test plan

• cargo test -p bevy_scene_macros
• cargo test -p bevy_scene enum_variant_subexpressions
• enum_variant_field_values_use_implicit_into still passes
• cargo fmt on changed crates