Replaces the string-based `tones` field with a type-safe
`buttons: list[KeypadEntry]` on `OutputDTMFFrame` and
`OutputDTMFUrgentFrame`, matching the existing singular `button`
field on `InputDTMFFrame`. A `from_string` classmethod builds the
list from a dial string like `"123#"` (invalid characters raise
ValueError from the `KeypadEntry` constructor).
The base output audio fallback now iterates `frame.buttons`
directly, LiveKit sends `frame.buttons[0].value`, and the Daily
transport joins the button values into the single string Daily's
`send_dtmf` expects.
d8b0e78bc8