fix: preserve partial content when synthesis stream drops mid-response

When the final synthesis stream dropped mid-way (connection error,
timeout), the onError handler discarded the streaming message entirely
and showed only the error. If the model had already streamed 90% of a
good answer, the user lost all of it.

Now: if the streaming message has partial content, it is preserved and
finalized with an interrupted note appended. The error is still shown
as a separate message so the user knows what happened.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

src/components/layout/ChatWorkspace/index.tsx

@@ -1571,13 +1571,35 @@ export function ChatWorkspace() {
       }
     },
     onError: (err: Error) => {
-      // Clean up streaming placeholder if the stream errored
+      // Clean up streaming placeholder if the stream errored.
+      // If the streaming message already has partial content (model streamed
+      // most of the answer before the connection dropped), preserve it and
+      // finalize it instead of discarding — losing a 90% complete response
+      // is worse than showing it with an error note appended.
       streamCancelRef.current = null
       const streamMsgId = streamingMsgIdRef.current
       streamingMsgIdRef.current = null
       clearBridgeDraft(sessionId)
       if (streamMsgId) {
-        setMessages(prev => prev.filter(m => m.id !== streamMsgId))
+        let preserved = false
+        setMessages(prev => {
+          const streamMsg = prev.find(m => m.id === streamMsgId)
+          if (streamMsg?.content && streamMsg.content.trim().length > 0) {
+            // Preserve partial content — finalize the streaming message
+            preserved = true
+            return prev.map(m =>
+              m.id === streamMsgId
+                ? { ...m, isStreaming: false, content: m.content + '\n\n---\n*Response interrupted — connection dropped before completion.*' }
+                : m,
+            )
+          }
+          // No useful content — remove the empty placeholder
+          return prev.filter(m => m.id !== streamMsgId)
+        })
+        if (preserved) {
+          if (err.name === 'AbortError') return
+          // Still show the error as a separate message so the user knows what happened
+        }
       }
       if (err.name === 'AbortError') return
       const isTimeout = err.name === 'TimeoutError'