Introduces a new `tones` field on `OutputDTMFFrame` and
`OutputDTMFUrgentFrame` for sending multi-digit DTMF sequences and
deprecates the existing single-key `button` field. When only `button`
is set, it is used as a single-character `tones` string for backward
compatibility.
`DTMFFrame` is kept as an empty marker class so both input and output
DTMF frames can still be identified via isinstance. `InputDTMFFrame`
keeps its required `button` field (single keypress semantics).
The Daily-specific `DailyOutputDTMFFrame` and
`DailyOutputDTMFUrgentFrame` frames no longer need to override
`button` and simply add `session_id` and `digit_duration_ms`, which
are forwarded to Daily's `send_dtmf` as `sessionId` and
`digitDurationMs`.
The base output audio fallback now iterates `tones` and generates a
tone per character; LiveKit's native DTMF path sends `tones[0]` since
its API is single-tone.
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