Fix Dyad Pro error messaging (#1884)

Addresses issue in https://github.com/dyad-sh/dyad/issues/1849

<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Fixes Pro users seeing an “Upgrade to Dyad Pro” prompt on provider
rate-limit errors and cleans up noisy fallback details in error
messages. Pro users now get the correct message, and errors are shorter
and clearer.

- **Bug Fixes**
- Show the upgrade link only when not on Pro for rate-limit errors
(Gemini and generic provider errors).
- Trim verbose fallback lists by splitting on "Fallbacks=[{" to keep the
fallback model’s error while removing the rest.

<sup>Written for commit e0f69dcda4b4aa0f1be2af3b1af163d84e995fe7.
Summary will update automatically on new commits.</sup>

<!-- End of auto-generated description by cubic. -->

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Avoids Pro upsell for Pro users on rate-limit errors and trims
fallback details more precisely in error messages.
> 
> - **Chat error handling (`src/components/chat/ChatErrorBox.tsx`)**:
> - **Rate-limit messaging**: Show Dyad Pro upgrade link only when
`!isDyadProEnabled` (prevents upsell for existing Pro users).
> - **Fallback error trimming**: Truncate error text at `"Fallbacks=[{"`
instead of `"Fallbacks="` to retain fallback model error context.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
e0f69dcda4b4aa0f1be2af3b1af163d84e995fe7. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

src/components/chat/ChatErrorBox.tsx

@@ -36,10 +36,15 @@ export function ChatErrorBox({
 
   // Important, this needs to come after the "free quota tier" check
   // because it also includes this URL in the error message
+  //
+  // Sometimes Dyad Pro can return rate limit errors and we do not want to
+  // show the upgrade to Dyad Pro link in that case because they are
+  // already on the Dyad Pro plan.
   if (
-    error.includes("Resource has been exhausted") ||
+    !isDyadProEnabled &&
+    (error.includes("Resource has been exhausted") ||
       error.includes("https://ai.google.dev/gemini-api/docs/rate-limits") ||
-    error.includes("Provider returned error")
+      error.includes("Provider returned error"))
   ) {
     return (
       <ChatErrorContainer onDismiss={onDismiss}>
@@ -93,8 +98,13 @@ export function ChatErrorBox({
     );
   }
   // This is a very long list of model fallbacks that clutters the error message.
-  if (error.includes("Fallbacks=")) {
+  //
-    error = error.split("Fallbacks=")[0];
+  // We are matching "Fallbacks=[{" and not just "Fallbacks=" because the fallback
+  // model itself can error and we want to include the fallback model error in the error message.
+  // Example: https://github.com/dyad-sh/dyad/issues/1849#issuecomment-3590685911
+  const fallbackPrefix = "Fallbacks=[{";
+  if (error.includes(fallbackPrefix)) {
+    error = error.split(fallbackPrefix)[0];
   }
   return (
     <ChatErrorContainer onDismiss={onDismiss}>