feat: add group AI spend endpoint (#27568)

## Description

Adds `GET /api/v2/groups/{group}/ai/spend`, returning the AI spend limit
and aggregate spend for a single group over the current budget period.
The period is derived from the deployment's configured budget period
rather than being caller-specified, matching the other AI spend
endpoints.

## Changes

- Add the `groupAISpend` handler and route, gated by the
`aigateway-cost-control` experiment and the `AIBridge` feature.
- Reuse the existing `GetOrganizationGroupsAISpend` query with a single
group ID, so no new query or authorization path is introduced.
- Add the `GroupAISpend` codersdk type and client method.

Closes
https://linear.app/codercom/issue/AIGOV-475/implement-apiv2groupsgroupaispend

> [!NOTE]
> Initially generated by Claude Opus 4.7, modified and reviewed by
@ssncferreira

enterprise/coderd/coderd.go

@@ -633,6 +633,15 @@ func New(ctx context.Context, options *Options) (_ *API, err error) {
 					)
 					r.Get("/", api.groupMembersAISpend)
 				})
+				r.Route("/ai/spend", func(r chi.Router) {
+					// AI cost controls are a paid feature (AI Governance add-on).
+					r.Use(
+						// TODO(AIGOV-443): remove once AI Gateway cost control functionality is stable.
+						httpmw.RequireExperiment(api.AGPL.Experiments, codersdk.ExperimentAIGatewayCostControl),
+						api.RequireFeatureMW(codersdk.FeatureAIBridge),
+					)
+					r.Get("/", api.groupAISpend)
+				})
 				r.Route("/ai/budget", func(r chi.Router) {
 					// AI cost controls are a paid feature (AI Governance add-on).
 					r.Use(api.RequireFeatureMW(codersdk.FeatureAIBridge))