Promotes two helpers that were living in pkg/server to pkg/api/stage.go,
alongside GetStage, ListFreightAvailableToStage, and the rest of the
Stage-domain operations. Both are now exported and unit-tested:

- StripStageForSummary: in-place projection that clears heavy fields
  (freightHistory[1..], promotionTemplate step configs, health.output)
  from a Stage. Previously a package-private helper in
  pkg/server/list_stages_v1alpha1.go.

- ListStageHealthOutputs: given a project and a set of Stage names,
  returns map[name]raw-health-output. Previously inlined (twice — once
  for the ConnectRPC handler and once for the REST handler) in
  pkg/server/get_stage_health_outputs_v1alpha1.go, alongside a
  private uniqueNonEmptyStrings helper that is now folded into the
  exported function.

The server handlers are now thin adapters that validate transport-level
concerns (batch-size cap) and delegate domain work to pkg/api. This
matches the convention akuity#6163 establishes for StageMatchesAnyWarehouse /
ListStagesByWarehouses.

Signed-off-by: Jacob Boykin <boykinmusic@gmail.com>
Signed-off-by: Jacob Boykin <jacob.boykin@akuity.io>

…n/stage-summary-api

Applies Kent's pkg/api Stage-domain consolidation ahead of its merge so
this branch is ready to land cleanly once akuity#6163 is in main.

Kent's PR promotes stageMatchesAnyWarehouse + listStagesByWarehouses
from pkg/server into pkg/api (exported, unit-tested, options-struct
shaped). Our REST listStages handler now calls api.ListStagesByWarehouses
directly. Our ConnectRPC ListStages handler does its list inline so it
can surface list.ResourceVersion for the list-then-watch response field;
filtering still goes through api.StageMatchesAnyWarehouse to keep the
matching logic in one place.

Signed-off-by: Jacob Boykin <boykinmusic@gmail.com>
Signed-off-by: Jacob Boykin <jacob.boykin@akuity.io>