fix(chat): clearer provider errors and no empty bubble, found with real models

- An error object sent inside the stream (OpenRouter's { code, message })
  showed as "[object Object]"; its message and status code are read now.
- A problem+json "detail" is added to the message: NVIDIA only said
  "Gone" for a retired model. 410 counts as model not found.
- "Cannot connect to API" from the SDK gets the connection hint.
- Text that is only whitespace (Kimi K2.6 sends a space before a tool
  call) no longer shows an empty bubble.
- allowSystemInMessages stops the warning on every request. Our system
  messages carry cache points; a client's own system messages are already
  dropped by the empty-content filter.

lib/llm-errors.ts

@@ -62,6 +62,8 @@ const STATUS_CODES: Record<number, LLMErrorCode> = {
     403: "forbidden",
     404: "model_not_found",
     408: "timeout",
+    // A retired model
+    410: "model_not_found",
     413: "context_too_long",
     429: "rate_limited",
 }
@@ -76,7 +78,10 @@ const GENERAL_TEXTS: Array<[RegExp, LLMErrorCode]> = [
         "invalid_api_key",
     ],
     [/rate limit|too many requests/i, "rate_limited"],
-    [/ECONNREFUSED|ENOTFOUND|ECONNRESET|fetch failed/i, "cannot_connect"],
+    [
+        /Cannot connect to API|ECONNREFUSED|ENOTFOUND|ECONNRESET|ETIMEDOUT|fetch failed/i,
+        "cannot_connect",
+    ],
 ]
 
 /** Secrets a provider may echo back: API keys, Bearer tokens, key=value */
@@ -91,6 +96,15 @@ function redact(text: string): string {
         )
 }
 
+function problemDetail(body: string): string | undefined {
+    try {
+        const detail = JSON.parse(body)?.detail
+        return typeof detail === "string" ? detail : undefined
+    } catch {
+        return undefined
+    }
+}
+
 /**
  * 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.
@@ -106,13 +120,28 @@ export function isToolCallError(error: unknown): boolean {
 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)
+    // Errors sent inside the stream can be plain objects like OpenRouter's
+    // { code: 503, message }
+    const plain = e as {
+        message?: unknown
+        code?: unknown
+        statusCode?: number
+    }
+    const raw =
+        e instanceof Error
+            ? e.message
+            : typeof plain?.message === "string"
+              ? plain.message
+              : String(e)
     const body = APICallError.isInstance(e) ? (e.responseBody ?? "") : ""
+    // A problem+json body names the reason the SDK left out (NVIDIA: "Gone")
+    const detail = problemDetail(body)
+    const message = redact(detail ? `${raw}: ${detail}` : raw).slice(0, 500)
     const text = `${raw} ${body}`
     const status = APICallError.isInstance(e)
         ? e.statusCode
-        : (e as { statusCode?: number })?.statusCode
+        : (plain?.statusCode ??
+          (typeof plain?.code === "number" ? plain.code : undefined))
 
     const find = (rules: Array<[RegExp, LLMErrorCode]>) =>
         rules.find(([pattern]) => pattern.test(text))?.[1]