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

lib/llm-errors.ts

@@ -0,0 +1,128 @@
+import {
+    APICallError,
+    InvalidToolInputError,
+    LoadAPIKeyError,
+    NoSuchToolError,
+    RetryError,
+    ToolCallRepairError,
+} from "ai"
+
+/**
+ * What went wrong with a model call, for a hint the user can act on. The
+ * provider's own message always goes along, because a guess can be wrong.
+ */
+export type LLMErrorCode =
+    | "invalid_api_key"
+    | "forbidden"
+    | "model_not_found"
+    | "insufficient_quota"
+    | "rate_limited"
+    | "context_too_long"
+    | "images_unsupported"
+    | "tools_unsupported"
+    | "output_truncated"
+    | "provider_unavailable"
+    | "cannot_connect"
+    | "timeout"
+    | "unknown"
+
+export interface LLMError {
+    type: "provider"
+    code: LLMErrorCode
+    message: string
+}
+
+// Texts that name the cause more precisely than the status code: a quota
+// error can come as 403 or 429, a context or image error as a plain 400
+const SPECIFIC_TEXTS: Array<[RegExp, LLMErrorCode]> = [
+    [
+        /context length|context window|maximum context|prompt is too long|input is too long|too many (?:input )?tokens/i,
+        "context_too_long",
+    ],
+    [
+        /image content block|image_url|does not support image|image input is not supported/i,
+        "images_unsupported",
+    ],
+    [
+        /does not support tools|tool use is not supported|tools? (?:are|is) not supported|function calling is not supported/i,
+        "tools_unsupported",
+    ],
+    // Bedrock, when the output limit cut the tool call's JSON short
+    [/toolUse\.input is invalid/i, "output_truncated"],
+    [
+        /insufficient[_ ]quota|insufficient balance|exceeded your current quota|credit balance is too low|余额不足/i,
+        "insufficient_quota",
+    ],
+]
+
+const STATUS_CODES: Record<number, LLMErrorCode> = {
+    401: "invalid_api_key",
+    402: "insufficient_quota",
+    // Not "invalid key": a valid key can lack access to a model or region
+    403: "forbidden",
+    404: "model_not_found",
+    408: "timeout",
+    413: "context_too_long",
+    429: "rate_limited",
+}
+
+const GENERAL_TEXTS: Array<[RegExp, LLMErrorCode]> = [
+    [
+        /model[_ ]not[_ ]found|model .*does not exist|unknown model|no such model/i,
+        "model_not_found",
+    ],
+    [
+        /invalid[_ ]api[_ ]key|incorrect api key|unauthorized/i,
+        "invalid_api_key",
+    ],
+    [/rate limit|too many requests/i, "rate_limited"],
+    [/ECONNREFUSED|ENOTFOUND|ECONNRESET|fetch failed/i, "cannot_connect"],
+]
+
+/** Secrets a provider may echo back: API keys, Bearer tokens, key=value */
+function redact(text: string): string {
+    return text
+        .replace(/\b(sk|pk|rk|ak)-[A-Za-z0-9_-]{8,}/g, "$1-[redacted]")
+        .replace(/\bBearer\s+[A-Za-z0-9._~+/-]+=*/gi, "Bearer [redacted]")
+        .replace(/\bAKIA[0-9A-Z]{16}\b/g, "[redacted]")
+        .replace(
+            /\b(api[_-]?key|access[_-]?key|secret|token|password|signature)(["']?\s*[:=]\s*["']?)[^\s"',&}]+/gi,
+            "$1$2[redacted]",
+        )
+}
+
+/**
+ * Model and tool errors the SDK sends back to the model as the tool result,
+ * so it can fix its call. Their text has to stay as it is.
+ */
+export function isToolCallError(error: unknown): boolean {
+    return (
+        InvalidToolInputError.isInstance(error) ||
+        NoSuchToolError.isInstance(error) ||
+        ToolCallRepairError.isInstance(error)
+    )
+}
+
+export function classifyLLMError(error: unknown): LLMError {
+    // After the SDK's retries, the last attempt says what happened
+    const e = RetryError.isInstance(error) ? error.lastError : error
+    const raw = e instanceof Error ? e.message : String(e)
+    const message = redact(raw).slice(0, 500)
+    const body = APICallError.isInstance(e) ? (e.responseBody ?? "") : ""
+    const text = `${raw} ${body}`
+    const status = APICallError.isInstance(e)
+        ? e.statusCode
+        : (e as { statusCode?: number })?.statusCode
+
+    const find = (rules: Array<[RegExp, LLMErrorCode]>) =>
+        rules.find(([pattern]) => pattern.test(text))?.[1]
+    const code =
+        (e instanceof Error && e.name === "TimeoutError" && "timeout") ||
+        (LoadAPIKeyError.isInstance(e) && "invalid_api_key") ||
+        find(SPECIFIC_TEXTS) ||
+        (status && STATUS_CODES[status]) ||
+        (status && status >= 500 && "provider_unavailable") ||
+        find(GENERAL_TEXTS) ||
+        "unknown"
+    return { type: "provider", code, message }
+}