Publish DTMF Helper for Local Participant

[ayukumar261]

Problem

The JavaScript, Rust, Python, and Node SDKs each have a publishDtmf helper that builds a SipDTMF packet and always sends it over the reliable data channel. The Go SDK has no equivalent, so developers have to build the packet themselves and remember to pass WithDataPublishReliable(true).

That matters because PublishDataPacket has defaulted to the lossy channel since #421 (v2.1.0), and the lossy channel has been unordered since #659 (v2.8.0). Digits sent with the default can be dropped or arrive out of order.

Changes

• Added PublishDTMF(code, digit, opts...) to LocalParticipant. It wraps PublishDataPacket and appends WithDataPublishReliable(true) after the caller's options, so a SipDTMF packet always goes over the reliable channel, even if the caller passes WithDataPublishReliable(false).
• Updated a stale comment above the default in PublishDataPacket. It said the Go default matches the protobuf default. That was true when Accept and send DTMF via data channel #415 wrote it, since both defaulted to reliable. Revert default behavior change with data publishing #421 flipped the Go default to lossy but left the comment, and the protobuf zero value is still RELIABLE, so the two have disagreed since then.

Testing

TestPublishDTMF publishes 1 2 3 # with WithDataPublishReliable(false) and asserts the subscriber receives four *livekit.SipDTMF packets in order, and that the publisher's WebRTC GetStats show four messages on the _reliable data channel and none on _lossy. Removing the override from the helper fails the test.

Notes

• Other options such as WithDataPublishDestination pass through unchanged.
• PublishDTMF mirrors the JavaScript and Rust SDKs (Python and Node wrap Rust).
• mage test, which is what CI runs and already includes -race, passes locally.

[CLAassistant]

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