Add the Orbit start-trial mutation

What does this MR do and why?

An Owner of a top-level group can now start an Orbit trial. A later MR connects the Orbit button to it.

• orbitStartTrial starts the CustomersDot trial and turns on Orbit, in one call.
• With the Orbit trial flag on, orbitUpdate turns on Orbit only for beta groups and active trials.
• Owners can start a trial. Beta groups cannot, because they already have Orbit for free.

Case	Result
Eligible group	The trial starts and Orbit turns on
The trial already exists and is active	Same result, so the call is safe to repeat. No second orbit_trial_started event.
The trial has ended	"Your GitLab Orbit trial has ended."
CustomersDot rejects the group, or does not know it	"This group is not eligible for a GitLab Orbit trial." The internal event records the CustomersDot reason.
CustomersDot does not answer	"GitLab Orbit is not available right now. Try again later."
Beta group, Maintainer, or subgroup	Access error
orbitUpdate on a group whose trial has ended	"Your GitLab Orbit trial has ended."
orbitUpdate on a group with no trial (eligible or ineligible)	"GitLab Orbit needs an active trial for this group."

References

https://gitlab.com/gitlab-org/orbit/-/work_items/19

Closes https://gitlab.com/gitlab-org/orbit/knowledge-graph/-/work_items/1337

Builds on !258297 (merged), !258545 (merged), and !258586 (merged).

How to set up and validate locally

With a local CustomersDot, in the Rails console:

Feature.enable(:orbit_trial, group)

Then, as an Owner of the group:

mutation {
  orbitStartTrial(input: { groupPath: "gitlab-org" }) {
    errors
    group { knowledgeGraphEnabled orbitTrial { state expiresOn } }
  }
}

Response

{
  "data": {
    "orbitStartTrial": {
      "errors": [],
      "group": {
        "knowledgeGraphEnabled": true,
        "orbitTrial": { "state": "ACTIVE", "expiresOn": "2026-10-31" }
      }
    }
  }
}

Demo against a local CustomersDot: an eligible group, start (Orbit turns on), start again (409, same result), a group that CustomersDot rejects (422), a beta group that cannot start a trial, Orbit off after the trial ends, and orbitUpdate that cannot turn it on again.

MR acceptance checklist

See the MR acceptance checklist.