refactor(coderd): add budget.CurrentPeriod for [start, end) windows (#26972)

## Description

Extracts the AI budget period computation into a shared
`budget.CurrentPeriod` helper. Pure refactor with no wire-value changes.

## Changes

- Add `budget.CurrentPeriod(now, period)` returning a
`PeriodWindow{Start, End}` in UTC. Unknown periods return an error,
matching the pattern used by `ResolveUserAIBudget` for unknown policies.
- Update the callers and respective tests to use `CurrentPeriod`.

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

coderd/aibridgedserver/aibridgedserver_test.go

@@ -27,6 +27,7 @@ import (
 	"cdr.dev/slog/v3"
 	"cdr.dev/slog/v3/sloggers/slogjson"
 	"cdr.dev/slog/v3/sloggers/slogtest"
+	"github.com/coder/coder/v2/coderd/aibridge/budget"
 	"github.com/coder/coder/v2/coderd/aibridged"
 	"github.com/coder/coder/v2/coderd/aibridged/proto"
 	"github.com/coder/coder/v2/coderd/aibridgedserver"
@@ -567,7 +568,9 @@ func TestIsBudgetExceeded(t *testing.T) {
 
 			req := &proto.IsBudgetExceededRequest{UserId: userIDStr}
 			if !tc.omitPeriodStart {
-				req.PeriodStart = timestamppb.New(dbtime.StartOfMonth(dbtime.Now().UTC()))
+				window, err := budget.CurrentPeriod(dbtime.Now(), codersdk.AIBudgetPeriodMonth)
+				require.NoError(t, err)
+				req.PeriodStart = timestamppb.New(window.Start)
 			}
 			resp, err := srv.IsBudgetExceeded(t.Context(), req)
 			if tc.wantErrContains != "" {