E2E Test: data channel send/receive test.

[cloudwebrtc]

Summary

Adds end-to-end pytests for LiveKit's data-channel (publish_data) routing semantics, covering targeted delivery, broadcast, topic propagation, and topic-on-targeted sends. Each scenario uses dedicated rtc.Room instances per participant and asserts that only the intended receivers see the packet.

Test	What it covers
test_data_one_to_one	destination_identities=["receiver"] reaches only receiver; bystander gets nothing.
test_data_one_to_many_targeted	destination_identities=["r1", "r2"] reaches both; an excluded participant gets nothing.
test_data_broadcast	Empty destination_identities fan-outs to every other participant in the room.
test_data_topic_passthrough	topic is preserved end-to-end; receiver-side filtering by topic works.
test_data_targeted_with_topic	Targeted send carries topic; non-targets receive nothing.

Implementation notes:

• _DataCollector subscribes to data_received and (optionally) filters by sender identity, so cross-test packet noise is impossible.
• _ensure_visible waits until the sender's room state lists the target identities — without this, targeted publishes can race the participant-joined signal and silently drop.
• _assert_no_data uses a 1s settle window before asserting absence, to avoid false negatives from in-flight delivery.

Requires

• LIVEKIT_URL, LIVEKIT_API_KEY, LIVEKIT_API_SECRET env vars (tests are auto-skipped otherwise).

Test plan

• pytest livekit-rtc/tests/test_dc.py -v passes against a current LiveKit server (verified locally on 1.10.1).
• Tests are skipped cleanly when LiveKit env vars are missing.
• No regressions in the existing tests/ suite.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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