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

coderd/aibridged/http.go

@@ -7,16 +7,12 @@ import (
 
 	"github.com/google/uuid"
 	"golang.org/x/xerrors"
-	"google.golang.org/protobuf/types/known/timestamppb"
 
 	"cdr.dev/slog/v3"
 	"github.com/coder/coder/v2/aibridge"
 	"github.com/coder/coder/v2/aibridge/recorder"
 	agplaibridge "github.com/coder/coder/v2/coderd/aibridge"
-	"github.com/coder/coder/v2/coderd/aibridge/budget"
 	"github.com/coder/coder/v2/coderd/aibridged/proto"
-	"github.com/coder/coder/v2/coderd/database/dbtime"
-	"github.com/coder/coder/v2/codersdk"
 )
 
 var _ http.Handler = &Server{}
@@ -149,15 +145,8 @@ func (s *Server) ServeHTTP(rw http.ResponseWriter, r *http.Request) {
 	}
 	logger = logger.With(slog.F("user_id", id))
 
-	periodWindow, err := budget.CurrentPeriod(dbtime.Now(), codersdk.AIBudgetPeriodMonth)
-	if err != nil {
-		logger.Warn(ctx, "compute AI budget period", slog.Error(err))
-		http.Error(rw, ErrBudgetCheck.Error(), http.StatusInternalServerError)
-		return
-	}
 	budgetResp, err := client.IsBudgetExceeded(ctx, &proto.IsBudgetExceededRequest{
-		UserId:      id.String(),
-		PeriodStart: timestamppb.New(periodWindow.Start),
+		UserId: id.String(),
 	})
 	if err != nil {
 		logger.Warn(ctx, "user AI budget check failed", slog.Error(err))