fix(web): use isFirstMessage instead of hardcoded false for invalid data

The onData callback's second parameter is isFirstMessage, not isError.
When encountering invalid response data (null or non-object), we should
preserve the current isFirstMessage state rather than hardcoding false.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

web/service/base.ts

@@ -218,7 +218,7 @@ export const handleStream = (
               return
             }
             if (!bufferObj || typeof bufferObj !== 'object') {
-              onData('', false, {
+              onData('', isFirstMessage, {
                 conversationId: undefined,
                 messageId: '',
                 errorMessage: 'Invalid response data',