✨ feat(amp-youtube): add data-channelid support for channel embeds

[pranav200408]

✨ New feature

Why
amp-youtube currently supports data-videoid and data-live-channelid, but there’s no direct way to embed a YouTube Channel. This adds first-class support for channel embeds to improve author experience and cover a common use case.

What

• Introduces a new attribute: data-channelid.
• Exactly one of data-videoid, data-live-channelid, or data-channelid is required; runtime asserts on conflicts.
• When data-channelid is present, amp-youtube uses the YouTube live_stream endpoint to render the channel.
• Handles attribute mutations: switches player source appropriately.

Linked issue
Closes #26304

Screenshots/Manual verification
Example:

Resulting iframe src contains:
.../embed/live_stream?channel=UC_x5XG1OV2P6uZZ5FSM9Ttw&enablejsapi=1&amp=1

Follow-ups included/remaining
📖 Docs: Add data-channelid to amp-youtube.md with example
✅ Tests: Unit tests for URL generation, exclusivity rule, and mutation behavior
🏗 Validator: Allow data-channelid and enforce “exactly one of” among data-videoid, data-live-channelid, data-channelid

Breaking changes
None

Risk/rollout
Low. New attribute; existing behavior for data-videoid and data-live-channelid unchanged.

Testing plan
Added/Will add unit tests.
Manually verified with a local example page; inspected iframe src and runtime assertions.

Checklist
✅ Code compiles locally
✅ Tests added/updated
✅ Documentation updated
✅ Validator config updated
✅ Linked to issue and used auto-close keyword

[CLAassistant]

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