feat(nimbus): add rollout setup progress and issues modal

[RJAK11]

Because

• users need to know their rollout setup progress

This commit

• adds a setup completion progress bar to the rollout sidebar
• adds an issues modal that lists invalid fields with links to the relevant card

Fixes #15927

Screen.Recording.2026-07-08.at.3.01.38.PM.mov

[RJAK11]

When testing locally with the "this is a rollout" checkbox checked, you'll hit 3 additional warnings not in the video for the population percent, total enrolled clients, and reference branch fields.

A few thoughts on handling these:

• Total enrolled clients doesn't apply to the new UI, and I was told we can just default it to 1 when we add the "create new rollout" flow, until we create a dedicated rollout serializer. I think the same idea works for Reference branch.
• For Population percent, the current serializer doesn't support the new rollout phases model, so I was thinking maybe we could also hardcode a value here too since it'll get overwritten anyway once the user requests to start a phase.

Separately, we might need to bring over the AI risk field from the old UI into the new Risks card, and Public description into the Overview card or we can handle them the same way since we don’t currently have those fields. Let me know what you think!

[RJAK11]

Update: instead of hard-coding some values to prevent the warnings mentioned previously, I added a dedicated NimbusRolloutReviewSerializer that the new UI uses (the old UI keeps the base serializer, so this is all scoped to the new UI). That clears the 3 warnings:

• Total enrolled clients and Reference branch: changed to allow empty values for rollouts (since neither seems to apply to the new UI).
• Population percent: allows 0% since population now comes from the rollout phases model. To keep that meaningful, I added schedule validation (>= 1 phase, first phase non-zero) that shows up as a setup issue on the Rollout schedule card.

However, we may still want to add AI risk in the Risks card and Public description in the Overview card.

[yashikakhurana]

Thank you @RJAK11 this is really great, nice approach to use separate serializer, and I have tested it locally, majorly works fine, some improvements to make, otherwise very good work