feat(errors): classify provider errors and show a hint the user can act on

- lib/llm-errors.ts sorts an error into about a dozen kinds (key
  rejected, no access, unknown model, no credit, rate limited, context
  too long, no image input, no tool calls, output cut off, provider down,
  cannot connect, timeout): first texts that name the cause precisely,
  then the HTTP status code, then general texts. It unwraps RetryError and
  hides keys and Bearer tokens in the provider's message
- The chat route uses it for errors before the stream and, through
  toUIMessageStreamResponse's onError, for errors in the stream. Errors
  of the model's own tool call stay as they are: the same text goes back
  to the model so it can fix the call
- The chat shows the hint in the user's language, then the provider's
  message; a rejected key, missing access or unknown model adds an "Open
  model settings" button. The Test button shows the same hints
- Fixes: our message "API key is required when using a custom base URL"
  was replaced by "Authentication failed" because it contains "key"; a
  provider's "Rate limit exceeded" opened this site's quota toast; an
  error body like {"error": ...} was shown as raw JSON; the Test button
  matched "401" in the message, where providers rarely put it
- Remove the string matching fallbacks in the chat panel

tests/e2e/chat-errors.spec.ts

@@ -0,0 +1,77 @@
+import { expect, type Page, test } from "@playwright/test"
+import { getIframe, sendMessage } from "./lib/fixtures"
+
+/** Answer the chat request with the given response and send a message */
+async function chatWith(
+    page: Page,
+    response: { status: number; contentType: string; body: string },
+) {
+    await page.route("**/api/chat", (route) => route.fulfill(response))
+    await page.goto("/", { waitUntil: "networkidle" })
+    await getIframe(page).waitFor({ state: "visible", timeout: 30000 })
+    await sendMessage(page, "Draw a box")
+}
+
+test("a rejected key shows a hint, the provider's words and a settings button", async ({
+    page,
+}) => {
+    await chatWith(page, {
+        status: 401,
+        contentType: "application/json",
+        body: JSON.stringify({
+            type: "provider",
+            code: "invalid_api_key",
+            message: "Authentication Fails, Your api key is invalid",
+        }),
+    })
+    await expect(
+        page.getByText("The provider rejected the API key"),
+    ).toBeVisible({
+        timeout: 15000,
+    })
+    // First match: the Next.js dev overlay at the end also lists the error
+    await expect(page.getByText("Authentication Fails").first()).toBeVisible()
+    await page.getByRole("button", { name: "Open model settings" }).click()
+    await expect(
+        page.getByRole("dialog", { name: "AI Model Configuration" }),
+    ).toBeVisible()
+})
+
+test("an error before the stream shows its text, not raw JSON", async ({
+    page,
+}) => {
+    await chatWith(page, {
+        status: 400,
+        contentType: "application/json",
+        body: JSON.stringify({
+            error: 'Model "x" is not available on the server',
+        }),
+    })
+    // The whole message, not the JSON around it (the Next.js dev overlay
+    // also lists the raw console error; it is not part of the app)
+    await expect(
+        page.getByText('Model "x" is not available on the server', {
+            exact: true,
+        }),
+    ).toBeVisible({ timeout: 15000 })
+})
+
+test("a provider rate limit is not shown as this site's quota", async ({
+    page,
+}) => {
+    const errorText = JSON.stringify({
+        type: "provider",
+        code: "rate_limited",
+        message: "Rate limit exceeded for your organization",
+    })
+    await chatWith(page, {
+        status: 200,
+        contentType: "text/event-stream",
+        body: `data: {"type":"start"}\n\ndata: ${JSON.stringify({ type: "error", errorText })}\n\ndata: [DONE]\n\n`,
+    })
+    await expect(
+        page.getByText("The provider is limiting requests"),
+    ).toBeVisible({ timeout: 15000 })
+    // The site's own tokens-per-minute toast
+    await expect(page.getByText("Rate limit reached")).toHaveCount(0)
+})