fix: reject oversized checkout diffs (#2488)

* fix: reject oversized checkout diffs

* fix(diff): keep large previews from disconnecting clients

The physical socket limit is an OOM backstop, not a frame-size contract. Bound structured diff construction incrementally and report oversized previews without closing the client's socket.

* fix(app): preserve oversized diff state

* fix(diff): stay within relay frame limit

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Co-authored-by: Mohamed Boudra <boudra.moha@gmail.com>

packages/protocol/src/messages.ts

@@ -4128,6 +4128,8 @@ const CheckoutDiffSubscriptionPayloadSchema = z.object({
   cwd: z.string(),
   files: z.array(ParsedDiffFileSchema),
   error: CheckoutErrorSchema.nullable(),
+  // COMPAT(diffTooLarge): added in v0.2.4, keep optional until the daemon floor is v0.2.4.
+  diffTooLarge: z.boolean().optional(),
 });
 
 export const SubscribeCheckoutDiffResponseSchema = z.object({