fix: deduplicate terminal notices and distinguish cancellation

Keep cancellation and failure details in message status while preserving existing replies. Render cancellation neutrally, restore terminal metadata from history, and offer Edit again only when input can be restored.

Release note: Cancelled and failed requests show a single notice; cancellation no longer appears as an error, and Edit again accurately describes the recovery action.

src/shared/assistant-contracts.ts

@@ -395,6 +395,7 @@ export const conversationMessageSchema = z
     displayCaptureTruncated: z.boolean().optional(),
     createdAt: z.number().int().nonnegative(),
     state: z.enum(['streaming', 'complete', 'error']),
+    terminalStatus: z.enum(['failed', 'cancelled']).optional(),
     status: z.string().max(4_000).optional(),
     contextCompression:
       conversationContextCompressionMarkerSchema.optional(),