BrowseMediaSource: domain is now required

The BrowseMediaSource class in the media_source integration has been tightened up. The domain parameter is now a required str instead of str | None, and the special "list every media source" root node has moved to its own class, RootBrowseMediaSource.

Previously, domain was optional only to represent one edge case: the top-level node returned when browsing media-source:// with no specific source selected. That made the type hint misleading for the 99% case — every actual media source has a domain — and added a None branch that consumers had to think about. Splitting the root into its own class removes that branch.

What changed

• BrowseMediaSource.__init__ now requires domain: str.
• A new RootBrowseMediaSource class represents the root browse node listing all available media sources. It hardcodes domain=None and identifier=None and uses media-source:// as its content ID.
• media_source.async_browse_media() and MediaSourceItem.async_browse() now return BrowseMediaSource | RootBrowseMediaSource.

Impact on custom integrations

Most integrations don't need any changes. If you implement a media_source.py platform, you were already passing your own domain to BrowseMediaSource — that keeps working.

You only need to act if:

• You pass domain=None to BrowseMediaSource. This is no longer allowed. Set your integration domain instead.

• You call media_source.async_browse_media() and annotate the result. Update the type hint to BrowseMediaSource | RootBrowseMediaSource, or narrow with isinstance() before using domain-specific attributes:

  from homeassistant.components.media_source import (
      BrowseMediaSource,
      async_browse_media,
  )
  
  result = await async_browse_media(hass, media_content_id)
  if isinstance(result, BrowseMediaSource):
      # result.domain is guaranteed to be a str here
      ...

See the updated media source platform documentation for the full reference.