Merging mono tracks transfers Track 1's effect stack to the resulting stereo track

### Bug description

_No response_

### Steps to reproduce

1. Generate/import a mono clip on Track 1
2. Add some effects to the stack
3. Generate/import a mono clip on Track 2
4. Make sure Track 1 is selected
5. Make stereo track
6. Check the resulting track's effect stack

### Expected behavior

The resulting track's effect stack is clear.

### Actual behavior

The resulting track's effect stack duplicates that of Track 1

### Audacity Version

Audacity 3.7.8

### Operating system

OS independent

### Additional context

_No response_

Reproduce the issue in Audacity 3.7.8 by creating two mono tracks, adding effects to Track 1, and using Make stereo track with Track 1 selected. Trace the track-merging handling and add a regression test showing that the resulting stereo track has an empty effect stack.

Written by the indexing model from the issue text.