Concatenate video doesn't work from multi-output gallery items

## Expected Behavior
Successfully concatenate videos created using Veo with multiple outputs

## Actual Behavior
When concatenating videos that were created using Veo with multiple outputs, the gallery shows multiple videos to choose from. Regardless which video you choose, concatenation always uses the first video in the list.

## Steps to Reproduce the Problem

1. Create two (2) videos using text to Video. Choose 4 output each time.
2. Choose to concatenate
3. Choose anything but the first output from the multi-output asset listed

Start by reproducing the issue with Veo assets that have multiple outputs, then trace how the gallery selection is passed into the concatenation flow. Confirm that choosing any output uses that selected video rather than the first item, and verify the reported reproduction steps still work.

Written by the indexing model from the issue text.