refactor(confirmation): simplify approval type and tighten inject typing

[E66Crisp]

Commit description

1. Simplify the ToolUIPartApproval union: keep a “pending” branch that forbids approved/reason, and a “responded” branch with approved: boolean and optional reason; remove redundant variants to improve readability and maintainability.
2. Use inject(ConfirmationKey) in useConfirmationContext to avoid manual generics/defaults, keeping runtime behavior the same while improving type accuracy.