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

coderd/apidoc/docs.go

@@ -3400,6 +3400,42 @@ const docTemplate = `{
                 ]
             }
         },
+        "/api/v2/groups/{group}/ai/spend": {
+            "get": {
+                "description": "Returns the AI spend limit and aggregate spend for the group.",
+                "produces": [
+                    "application/json"
+                ],
+                "tags": [
+                    "Enterprise"
+                ],
+                "summary": "Get group AI spend",
+                "operationId": "get-group-ai-spend",
+                "parameters": [
+                    {
+                        "type": "string",
+                        "format": "uuid",
+                        "description": "Group ID",
+                        "name": "group",
+                        "in": "path",
+                        "required": true
+                    }
+                ],
+                "responses": {
+                    "200": {
+                        "description": "OK",
+                        "schema": {
+                            "$ref": "#/definitions/codersdk.GroupAISpend"
+                        }
+                    }
+                },
+                "security": [
+                    {
+                        "CoderSessionToken": []
+                    }
+                ]
+            }
+        },
         "/api/v2/groups/{group}/members": {
             "get": {
                 "produces": [
@@ -20642,6 +20678,33 @@ const docTemplate = `{
                 }
             }
         },
+        "codersdk.GroupAISpend": {
+            "type": "object",
+            "properties": {
+                "current_spend_micros": {
+                    "description": "CurrentSpendMicros is the group's spend over the current budget\nperiod.",
+                    "type": "integer"
+                },
+                "group_id": {
+                    "type": "string",
+                    "format": "uuid"
+                },
+                "period_end": {
+                    "description": "PeriodEnd is the exclusive upper bound of the current budget\nperiod.",
+                    "type": "string",
+                    "format": "date-time"
+                },
+                "period_start": {
+                    "description": "PeriodStart is the inclusive lower bound of the current budget\nperiod.",
+                    "type": "string",
+                    "format": "date-time"
+                },
+                "spend_limit_micros": {
+                    "description": "SpendLimitMicros is the group's configured AI spend limit. Null when\nthe group has no configured budget.",
+                    "type": "integer"
+                }
+            }
+        },
         "codersdk.GroupMemberAISpend": {
             "type": "object",
             "properties": {

coderd/apidoc/swagger.json

@@ -2999,6 +2999,38 @@
 				]
 			}
 		},
+		"/api/v2/groups/{group}/ai/spend": {
+			"get": {
+				"description": "Returns the AI spend limit and aggregate spend for the group.",
+				"produces": ["application/json"],
+				"tags": ["Enterprise"],
+				"summary": "Get group AI spend",
+				"operationId": "get-group-ai-spend",
+				"parameters": [
+					{
+						"type": "string",
+						"format": "uuid",
+						"description": "Group ID",
+						"name": "group",
+						"in": "path",
+						"required": true
+					}
+				],
+				"responses": {
+					"200": {
+						"description": "OK",
+						"schema": {
+							"$ref": "#/definitions/codersdk.GroupAISpend"
+						}
+					}
+				},
+				"security": [
+					{
+						"CoderSessionToken": []
+					}
+				]
+			}
+		},
 		"/api/v2/groups/{group}/members": {
 			"get": {
 				"produces": ["application/json"],
@@ -18790,6 +18822,33 @@
 				}
 			}
 		},
+		"codersdk.GroupAISpend": {
+			"type": "object",
+			"properties": {
+				"current_spend_micros": {
+					"description": "CurrentSpendMicros is the group's spend over the current budget\nperiod.",
+					"type": "integer"
+				},
+				"group_id": {
+					"type": "string",
+					"format": "uuid"
+				},
+				"period_end": {
+					"description": "PeriodEnd is the exclusive upper bound of the current budget\nperiod.",
+					"type": "string",
+					"format": "date-time"
+				},
+				"period_start": {
+					"description": "PeriodStart is the inclusive lower bound of the current budget\nperiod.",
+					"type": "string",
+					"format": "date-time"
+				},
+				"spend_limit_micros": {
+					"description": "SpendLimitMicros is the group's configured AI spend limit. Null when\nthe group has no configured budget.",
+					"type": "integer"
+				}
+			}
+		},
 		"codersdk.GroupMemberAISpend": {
 			"type": "object",
 			"properties": {