Embed: Update oEmbed regex for ReverbNation to support legacy subdomain

[yogeshbhutkar]

Trac ticket: https://core.trac.wordpress.org/ticket/66079

What, Why, and How?

ReverbNation moved their site and oEmbed API to legacy.reverbnation.com. The old www endpoint 404s and never redirects, so artist and song URLs stop embedding with the old URL. www pages still redirect to legacy, however, our matcher only allowed www, so a pasted legacy link never used the sanctioned provider. Legacy URLs didn't match the provider array, so they fell through to discovery via the page’s oEmbed <link> tags.

This PR updates the provider row: point the endpoint at legacy.reverbnation.com/oembed, and match both www and legacy so either hostname uses that API as a provider instead of discovery.

Screencast

screencast.mov

Use of AI Tools

AI assistance: Yes
Tool(s): Codex
Model(s): GPT-6 Astra
Used for: Validating the regex and running discovery on oEmbed endpoints for artists, songs, albums, and collections.

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