Extract the cross-rig bead formatting logic into a testable helper
function and add comprehensive unit tests:
- TestFormatTrackBeadID: 8 test cases covering HQ beads, cross-rig
beads, and edge cases (single segment, empty string, many segments)
- TestFormatTrackBeadIDConsumerCompatibility: 3 test cases verifying
the external ref format can be correctly parsed by consumers in
convoy.go, model.go, feed/convoy.go, and web/fetcher.go
The helper function includes godoc with examples showing expected
behavior for different bead ID formats.
