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

codersdk/aibridge.go

@@ -92,6 +92,13 @@ type OrganizationGroupAISpend struct {
 	CurrentSpendMicros int64 `json:"current_spend_micros"`
 }
 
+// GroupAISpend is the current AI spend snapshot for a single group within
+// the active budget period.
+type GroupAISpend struct {
+	AISpendPeriodWindow
+	OrganizationGroupAISpend
+}
+
 // GroupMembersAISpend reports per-member AI spend attributed to a specific
 // group in the active budget period.
 type GroupMembersAISpend struct {
@@ -577,6 +584,25 @@ func (c *Client) OrganizationGroupsAISpend(ctx context.Context, organization uui
 	return resp, json.NewDecoder(res.Body).Decode(&resp)
 }
 
+// GroupAISpend returns AI spend for the given group within the active budget
+// period.
+func (c *Client) GroupAISpend(ctx context.Context, group uuid.UUID) (GroupAISpend, error) {
+	res, err := c.Request(ctx, http.MethodGet,
+		fmt.Sprintf("/api/v2/groups/%s/ai/spend", group.String()),
+		nil,
+	)
+	if err != nil {
+		return GroupAISpend{}, xerrors.Errorf("make request: %w", err)
+	}
+	defer res.Body.Close()
+
+	if res.StatusCode != http.StatusOK {
+		return GroupAISpend{}, ReadBodyAsError(res)
+	}
+	var resp GroupAISpend
+	return resp, json.NewDecoder(res.Body).Decode(&resp)
+}
+
 // GroupMembersAISpend returns AI spend attributed to the given group for the
 // specified users within the active budget period. At most 100 user IDs may be
 // requested per call, and callers with more members are expected to batch