fix: add missing_key error kind for missing chat api_key_id (#25783)

Refs CODAGT-486

- `codersdk/chats.go`: New `ChatErrorKindMissingKey` constant and
`AllChatErrorKinds` entry
- `coderd/x/chatd/chaterror/message.go`: `terminalMessage` and
`retryMessage` cases
- `coderd/x/chatd/model_routing_aibridge.go`: Pre-classify error with
`WithClassification`
- `coderd/x/chatd/model_routing_internal_test.go`: Classification
assertion on production path (CRF-2)
- `chatStatusHelpers.ts`: Frontend title "Chat interrupted"
- `LiveStreamTail.stories.tsx`: Storybook story with `detail` assertion
- `docs/ai-coder/ai-gateway/clients/coder-agents.md`: Troubleshooting
entry
- Tests: classification round-trip, terminal message, metrics kind
enumeration

> Generated with [Coder Agents](https://coder.com/agents) on behalf of
@johnstcn

site/src/api/typesGenerated.ts

@@ -1959,6 +1959,7 @@ export type ChatErrorKind =
 	| "auth"
 	| "config"
 	| "generic"
+	| "missing_key"
 	| "overloaded"
 	| "rate_limit"
 	| "startup_timeout"
@@ -1969,6 +1970,7 @@ export const ChatErrorKinds: ChatErrorKind[] = [
 	"auth",
 	"config",
 	"generic",
+	"missing_key",
 	"overloaded",
 	"rate_limit",
 	"startup_timeout",

site/src/pages/AgentsPage/components/ChatConversation/LiveStreamTail.stories.tsx

@@ -193,6 +193,41 @@ export const TerminalTimeoutErrorUnknownProvider: Story = {
 	},
 };
 
+/** Missing API key shows the "Chat interrupted" terminal error. */
+export const TerminalMissingKeyError: Story = {
+	args: {
+		...defaultArgs,
+		liveStatus: buildLiveStatus({
+			streamError: {
+				kind: "missing_key",
+				message:
+					"This conversation was started with an API key that is no longer available. Send your message again to continue.",
+				retryable: false,
+				detail:
+					"If this error persists after resending, please report it as a bug.",
+			},
+		}),
+	},
+	play: async ({ canvasElement }) => {
+		const canvas = within(canvasElement);
+		expect(
+			canvas.getByRole("heading", { name: /chat interrupted/i }),
+		).toBeVisible();
+		expect(
+			canvas.getByText(
+				/this conversation was started with an api key that is no longer available/i,
+			),
+		).toBeVisible();
+		expect(
+			canvas.getByText(/if this error persists after resending/i),
+		).toBeVisible();
+		// Guard against the generic fallback.
+		expect(
+			canvas.queryByText(/the chat request failed unexpectedly/i),
+		).not.toBeInTheDocument();
+	},
+};
+
 /** Retrying a transport timeout shows attempt + countdown. */
 export const RetryingTimeoutAnthropic: Story = {
 	args: {

site/src/pages/AgentsPage/components/ChatConversation/chatStatusHelpers.ts

@@ -42,6 +42,8 @@ export const getErrorTitle = (
 			return "Configuration error";
 		case "usage_limit":
 			return "Usage limit reached";
+		case "missing_key":
+			return "Chat interrupted";
 		default:
 			return mode === "retry" ? "Retrying request" : "Request failed";
 	}