feat(error-handling): implement LLM error adaptation and classification for chat streaming

- Introduced LLMErrorCategory and adapt_llm_exception to normalize LLM exceptions.
- Updated llm_retryable_message and llm_permanent_message to utilize the new adaptation logic.
- Enhanced classify_stream_exception to classify provider errors and return user-friendly messages.
- Added tests for error classification and adaptation to ensure robustness.
- Updated frontend error handling to display appropriate messages based on new classifications.

surfsense_web/components/free-chat/free-chat-page.tsx

@@ -63,6 +63,21 @@ function normalizeFreeChatErrorMessage(error: unknown): string {
 	if (code === "THREAD_BUSY") {
 		return "A previous response is still stopping. Please try again in a moment.";
 	}
+	if (code === "MODEL_AUTH_FAILED") {
+		return "This model’s API key is invalid or expired. Switch models, or update the API key.";
+	}
+	if (code === "MODEL_NOT_FOUND") {
+		return "This model is unavailable or no longer exists. Please switch models.";
+	}
+	if (code === "MODEL_CONTEXT_LIMIT") {
+		return "This request is too large for the selected model. Reduce the input or switch models.";
+	}
+	if (code === "MODEL_PROVIDER_UNAVAILABLE") {
+		return "The selected model provider is temporarily unavailable. Please try again or switch models.";
+	}
+	if (code === "RATE_LIMITED") {
+		return "This model is temporarily rate-limited. Please try again in a few seconds or switch models.";
+	}
 	return error.message || "An unexpected error occurred";
 }
 
@@ -154,7 +169,7 @@ export function FreeChatPage() {
 			assistantMsgId: string,
 			signal: AbortSignal,
 			turnstileToken: string | null
-		): Promise<"captcha" | void> => {
+		): Promise<"captcha" | undefined> => {
 			const reqBody: Record<string, unknown> = {
 				model_slug: modelSlug,
 				messages: messageHistory,