Media player source selection

[jamesmulcahy]

Home Assistant media players have a list of sources (inputs) and can select one. This sends a media player's sources to the NSPanel and lets the panel select one.

A media player with a source volume strategy (#385) has two source lists: its own, and those of the source player feeding it. In an audio matrix setup that is the zone's inputs (Movie Theatre, James Matrix, …) and the Sonos Favorites of the speaker the zone is currently playing from.

Protobuf

• NSPanelEntityState.MediaPlayer.player_sources / source_player_sources: a SourceSelection with the sources, the current source and whether one can be selected. source_player_sources is unset when there is no source player.
• MediaPlayerCommand.select_player_source / select_source_player_source: select by name, empty means no change. By name because the source player's list changes completely when the zone switches source, so an index from an older state could select the wrong entry.
• current_source is often empty for the source player: Sonos only reports source for Line-in/TV, not for a playing favourite.
• The firmware C files were missing volume_step from Home Assistant media players and album art for NSPanels #385 and pick it up here.

Manager

• HomeAssistantMediaPlayer reads source_list / source and calls media_player.select_source.
• Following the source player now lives in a new HomeAssistantSourcePlayer, used by both source volume strategies. Before, only source_entity followed the source player; player_attributes only read the player's own attributes and so could not see the source player's sources. The strategies now only differ in where the source volume comes from. With player_attributes, the manager now also subscribes to the source player, which costs one Home Assistant state request per source change.
• When the player switches source, the previous source player's sources are dropped straight away instead of being offered until the new ones arrive.
• The NSPanel can only receive 4 KB in one MQTT message (larger messages are silently dropped by the firmware today), so at most 32 sources of up to 64 bytes are sent per list. Longer names are cut on a UTF-8 boundary, and the shortened name sent back by the panel is matched to the full name before selecting.

Not in this PR

• Panel UI: separate firmware PR. It needs to page through lists longer than 12.
• Tests: will follow on top of the MQTTManager test harness (Add MQTTManager unit test harness and CI #392).
• No web UI or REST changes; the existing source volume settings already identify the source player.

Status

Draft: compile-checked only so far, not yet built in Docker or run against Home Assistant.

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