Resolve runner_machine_id when queuing a suspend/resume build

Related MRs

1. !244035 (merged) — schema + read-side groundwork
2. !247560 (merged) — persist Ci::JobRuntimeEnvironment on suspend
3. !247561 (merged) — record the runner machine assignment when a build starts running
4. This MR — resolve runner_machine_id when queuing a suspend/resume build
5. !247563 — consume the routed runner machine when assigning jobs
6. !247564 — retire the old environment_key string-matching routing mechanism

What does this MR do and why?

When a suspended CI job resumes, the follow-up job has to land on the exact runner machine that ran the original job — that machine is the one holding the suspended environment's state. !247561 already records which runner machine ran a suspending build, onto that build's own environment row.

This MR carries that recorded id forward to the resuming job, at the moment it's queued. Ci::JobRuntimeEnvironment.runner_machine_id_for looks up whether any build tied to the same Ci::RuntimeEnvironment already has a runner machine recorded (there can be more than one prior build in the chain; it picks the earliest recorded one), and Ci::PendingBuild.args_from_build stores that id on the new build's queue entry (ci_pending_builds.runner_machine_id). Resolving it once here, instead of on every runner poll, is what lets the poll-time check become a plain indexed column filter instead of a per-poll lookup.

Gated behind ci_suspendable_environment_runner_routing, checked against the build's project; no-ops for builds that aren't part of a suspend/resume chain. This MR only makes the resolved id available on the queue entry — it doesn't change which runner actually gets offered the job. That happens once !247563 wires the runner-polling path to read this column.

Part of routing resumed builds to the runner machine that holds their suspended environment: Resumable Jobs for CI and Agent Sessions (gitlab-org#21159)

References

Depends on !247561 (merged) and !244035 (merged).

Issue: Resumable Jobs for CI and Agent Sessions (gitlab-org#21159)

Blueprint: Add blueprint for suspendable environments (gitlab-com/content-sites/handbook!18954 - merged)

How to set up and validate locally

Run the spec files covering this change:

bundle exec rspec spec/models/ci/job_runtime_environment_spec.rb spec/models/ci/pending_build_spec.rb

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