fix: generate-method skips trait impl blocks when picking insertion site

[Brumbelow]

Follow up to #21684

• The enclosing-impl shortcut accepted any impl whose self_ty matched the adt, so a call to a missing inherent method from inside a trait impl body appended the stub to the trait impl
• This produced code that does not compile (the trait does not declare the generated fn).

The fix:

• Tighten the filter with trait_(db).is_none(); fall back to the existing find_struct_impl path (or new-impl synthesis) for the trait-impl cursor case.
• Closes Assist "Generate method" selects current impl block even if it's a trait impl block #22123.

[Brumbelow]

I can remove the comments here too (if needed), just force of habit. LMK either way