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

codersdk/deployment.go

@@ -612,6 +612,15 @@ var AIBudgetPeriods = []string{
 	string(AIBudgetPeriodMonth),
 }
 
+// NewAIBudgetPeriodFromString converts s to an AIBudgetPeriod, falling back to
+// AIBudgetPeriodMonth when s is empty or not a recognized period.
+func NewAIBudgetPeriodFromString(s string) AIBudgetPeriod {
+	if slices.Contains(AIBudgetPeriods, s) {
+		return AIBudgetPeriod(s)
+	}
+	return AIBudgetPeriodMonth
+}
+
 // DeploymentValues is the central configuration values the coder server.
 type DeploymentValues struct {
 	Verbose             serpent.Bool   `json:"verbose,omitempty"`