feat: add user AI spend endpoint (#26978)

## Description

Adds the `GET /api/v2/users/{user}/ai/spend` endpoint returning the
user's current AI spend, effective budget, and period bounds.

## Changes

- Add `userAISpendStatus` handler under the same feature/experiment gate
as `/api/v2/users/{user}/ai/budget`.
- Add `codersdk.UserAIBudgetSummary` (embedded into `UserAISpendStatus`)
and a `UserAISpendStatus` client method.
- Move `LimitSource` from `coderd/aibridge/budget` to `codersdk` so the
type is shared across endpoints.

Closes https://linear.app/codercom/issue/AIGOV-472

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

docs/reference/api/enterprise.md

@@ -3777,6 +3777,49 @@ curl -X DELETE http://coder-server:8080/api/v2/users/{user}/ai/budget \
 
 To perform this operation, you must be authenticated. [Learn more](authentication.md).
 
+## Get user AI spend
+
+### Code samples
+
+```shell
+# Example request using curl
+curl -X GET http://coder-server:8080/api/v2/users/{user}/ai/spend \
+  -H 'Accept: application/json' \
+  -H 'Coder-Session-Token: API_KEY'
+```
+
+`GET /api/v2/users/{user}/ai/spend`
+
+### Parameters
+
+| Name   | In   | Type   | Required | Description              |
+|--------|------|--------|----------|--------------------------|
+| `user` | path | string | true     | User ID, username, or me |
+
+### Example responses
+
+> 200 Response
+
+```json
+{
+  "current_spend_micros": 0,
+  "effective_group_id": "85e2b926-ddfb-4c66-b68e-b66e5acec6c0",
+  "limit_source": "user_override",
+  "period_end": "2019-08-24T14:15:22Z",
+  "period_start": "2019-08-24T14:15:22Z",
+  "spend_limit_micros": 0,
+  "user_id": "a169451c-8525-4352-b8ca-070dd449a1a5"
+}
+```
+
+### Responses
+
+| Status | Meaning                                                 | Description | Schema                                                             |
+|--------|---------------------------------------------------------|-------------|--------------------------------------------------------------------|
+| 200    | [OK](https://tools.ietf.org/html/rfc7231#section-6.3.1) | OK          | [codersdk.UserAISpendStatus](schemas.md#codersdkuseraispendstatus) |
+
+To perform this operation, you must be authenticated. [Learn more](authentication.md).
+
 ## Get user quiet hours schedule
 
 ### Code samples