Fix Squeezelite output quality

[OzGav]

Previously, squeezelite players in sync groups were limited to
44.1kHz/16-bit output even when configured to support higher
sample rates and bit depths (e.g., 96kHz/24-bit). Individual
players worked correctly, but grouping them caused quality
degradation.

The issue was in _serve_multi_client_stream() which created
AudioFormat with only the content type, causing sample rate
and bit depth to default to minimum values.

This fix:

• Uses streams controller's get_output_format() to properly
  determine output format based on player capabilities
• Preserves original content format through MultiClientStream
• Passes content format to get_output_format() for proper
  quality negotiation

Grouped players now output at min(content_format, player_capability),
matching the behavior of individual players.

Fixes: music-assistant/server#<issue_number>