fix: defensive SDK response handling & parts-reader normalization

- Replace all response.data ?? [] with (response.data ?? response)
  pattern across 14 files to handle SDK array-shaped responses
- Normalize SDK parts in parts-reader.ts by injecting sessionID/
  messageID before validation (P1: SDK parts lack these fields)
- Treat unknown part types as having content in
  recover-empty-content-message-sdk.ts to prevent false placeholder
  injection on image/file parts
- Replace local isRecord with shared import in parts-reader.ts

src/features/background-agent/manager.ts

@@ -875,7 +875,7 @@ export class BackgroundManager {
         path: { id: sessionID },
       })
 
-      const messages = response.data ?? []
+      const messages = ((response.data ?? response) as unknown as Array<{ info?: { role?: string } }>) ?? []
       
       // Check for at least one assistant or tool message
       const hasAssistantOrToolMessage = messages.some(