refactor: use AI budget period from deployment config (#27117)

## Description

Read the AI budget period from the deployment config on both the RPC server and the `/users/{user}/ai/spend` endpoint, instead of hardcoding `month`. Drops the `period_start` RPC parameter that was incorrectly introduced in #26915: the period should have been derived from the deployment config from the start.

## Changes

- Add `codersdk.NewAIBudgetPeriodFromString`, mirroring `NewAIBudgetPolicyFromString`.
- `aibridgedserver.Server` takes a `quartz.Clock`, reads `BudgetPeriod` from the deployment config at construction, and computes the period window inside `IsBudgetExceeded`.
- Remove `period_start` from `IsBudgetExceededRequest` and stop sending it from the daemon.
- The `userAISpendStatus` endpoint reads the period from `AIBridgeConfig.BudgetPeriod` instead of hardcoding month.

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

enterprise/coderd/aibridge.go

@@ -890,7 +890,8 @@ func (api *API) userAISpendStatus(rw http.ResponseWriter, r *http.Request) {
 	user := httpmw.UserParam(r)
 	logger := api.Logger.With(slog.F("user_id", user.ID))
 
-	periodWindow, err := budget.CurrentPeriod(api.Clock.Now(), codersdk.AIBudgetPeriodMonth)
+	period := codersdk.NewAIBudgetPeriodFromString(api.DeploymentValues.AI.BridgeConfig.BudgetPeriod)
+	periodWindow, err := budget.CurrentPeriod(api.Clock.Now(), period)
 	if err != nil {
 		logger.Error(ctx, "failed to compute AI budget period", slog.Error(err))
 		httpapi.InternalServerError(rw, err)

enterprise/coderd/aibridgeserve.go

@@ -144,6 +144,7 @@ func (api *API) aiGatewayServe(rw http.ResponseWriter, r *http.Request) {
 		api.ExternalAuthConfigs,
 		api.AGPL.Experiments,
 		api.AGPL.AISeatTracker,
+		api.AGPL.Clock,
 	)
 	if err != nil {
 		if !xerrors.Is(err, context.Canceled) {