Epic: pageserver: updated detach API (enable doing ancestor detach on timelines that have more than one ancestor)

[jcsp]

Currently the timeline detach code can only run if a timeline has a single ancestor: it's harder to go gather layers from a stack of ancestors.

When archived branches are used like snapshots, it is important that after restoring from such a snapshot by creating a child branch, we can still detach it.

This is simplified a lot by the insight that the intervening snapshot branch is empty of writes, so all of the IO we do to copy layers can be sent to the ancestor.

When detaching such a branch, we also don't want to bring any children of the parent with us: this is different to when we're detaching a branch that was created via a PITR restore. This part's probably pretty simple, just skip doing it.

Tasks:

• Update ancestor detach code to work on branches that have two ancestors, and the immediate parent has no data.
• Make reparenting children of the parent branch optional via a boolean API flag.
• Revisit API return codes: to help control plane distinguish between "busy" and "stuck", we should return a different return code and/or include a quantitative progress in response body. Adding polling in cplane is https://github.com/neondatabase/cloud/issues/22362 , but that will just treat all 503 as "in progress" (via https://neondb.slack.com/archives/C03H1K0PGKH/p1738689273206469?thread_ts=1738678164.575429&cid=C03H1K0PGKH)

[skyzh]

On item 1: It makes sense to expand the detach ancestor code to handle some easy cases like this, it would be a quick patch.

On item 2: sounds like a quick patch as well.

On item 3: I think I'll start with making the current error code more meaningful (i.e., 50X as error, 50Y as detaching in progress), and in the future, it would make sense to have two APIs: start_detach (cplane calls once) and finish_detach (cplane polls until it returns 200). Tenant still remains accessible between start_detach and finish_detach versus cplane now blocks all operations once detach starts.

Anyways, all tasks sound lightweight and I think I can make some progress next week.