feat(site/src/pages/AISettingsPage/SpendPage): announce cost controls move to AI Governance (#27543)

Adds an informational banner to the AI settings **Spend** tab announcing
that cost controls features move to AI Governance in v2.37, with a link
to the AI Gateway cost controls docs.

Banner copy:

> Cost controls features will move to AI Governance in v2.37. [Read more
here](https://coder.com/docs/ai-coder/ai-gateway/cost-controls)

The link uses the existing `docs()` helper from `#/utils/docs`, so it
resolves the deployment's configured `docs-url` meta tag and otherwise
falls back to a version-pinned `coder.com/docs/@<version>` URL. This
matches how sibling AI settings pages link out (for example
`GatewayKeysPageView` and `ProvidersPageView`).

It points at `/ai-coder/ai-gateway/cost-controls`, the AI Gateway Cost
Controls page added in #27570. That page is present on `main` and the
URL resolves, so the banner links to live documentation.

The banner renders on the main Spend tab. It is intentionally not shown
in the per-user spend drill-in sub-view, which returns early from a
separate component.

## Validation

- Extended the existing `SpendWithLimitsAndUsers` story to assert the
banner copy and the resolved docs `href`. Both assertions were verified
to fail when the banner is removed and when the link points somewhere
else.
- Storybook story tests for `SpendPageView.stories.tsx` pass (12 tests),
plus `tsc -p .`, `biome check`, and `lint:compiler` clean.
- `make pre-commit` passed through the git hooks.

> Mux opened this PR on Mike's behalf.

site/src/pages/AISettingsPage/SpendPage/SpendPageView.stories.tsx

@@ -300,6 +300,17 @@ export const SpendWithLimitsAndUsers: Story = {
 		const canvas = within(canvasElement);
 
 		await canvas.findByText("Spend limits and usage");
+		await expect(
+			canvas.getByText(
+				/Cost controls features will move to AI Governance in v2\.37\./,
+			),
+		).toBeInTheDocument();
+		await expect(
+			canvas.getByRole("link", { name: /Read more here/ }),
+		).toHaveAttribute(
+			"href",
+			expect.stringContaining("/ai-coder/ai-gateway/cost-controls"),
+		);
 		await expect(
 			canvas.getByRole("switch", { name: "Spend limit" }),
 		).toBeInTheDocument();

site/src/pages/AISettingsPage/SpendPage/SpendPageView.tsx

@@ -1,9 +1,11 @@
 import type { FC } from "react";
 import { getErrorMessage } from "#/api/errors";
 import type * as TypesGen from "#/api/typesGenerated";
+import { Alert, AlertDescription } from "#/components/Alert/Alert";
 import { ErrorAlert } from "#/components/Alert/ErrorAlert";
 import { Button } from "#/components/Button/Button";
 import type { DateRangeValue } from "#/components/DateRangePicker/DateRangePicker";
+import { Link } from "#/components/Link/Link";
 import type { PaginationResult } from "#/components/PaginationWidget/PaginationContainer";
 import {
 	SettingsHeader,
@@ -22,6 +24,7 @@ import {
 	useTemporarySavedState,
 } from "#/components/TemporarySavedState/TemporarySavedState";
 import { dollarsToMicros, microsToDollars } from "#/utils/currency";
+import { docs } from "#/utils/docs";
 import {
 	DefaultLimitController,
 	type DefaultLimitFormValues,
@@ -232,6 +235,19 @@ export const SpendPageView: FC<SpendPageViewProps> = ({
 				>
 					{(userCtrl) => (
 						<div className="flex max-w-[1100px] flex-col gap-8">
+							<Alert severity="info">
+								<AlertDescription>
+									Cost controls features will move to AI Governance in v2.37.{" "}
+									<Link
+										href={docs("/ai-coder/ai-gateway/cost-controls")}
+										target="_blank"
+										rel="noreferrer"
+									>
+										Read more here
+									</Link>
+								</AlertDescription>
+							</Alert>
+
 							<SettingsHeader>
 								<SettingsHeaderTitle>
 									Spend limits and usage