fix(files): keep large source previews responsive (#3665)

Large read-only files bypassed the editor's virtualization and eagerly mounted every highlighted token, which could stall the renderer for seconds and consume gigabytes of memory. Route source previews through bounded platform renderers and reject reads above their display budgets before accumulation.

packages/protocol/src/messages.ts

@@ -2555,6 +2555,7 @@ export const FileExplorerRequestSchema = z.object({
   mode: z.enum(["list", "file"]),
   requestId: z.string(),
   acceptBinary: z.boolean().optional(),
+  maxBytes: z.number().int().positive().optional(),
 });
 
 export const FileVersionSchema = z.discriminatedUnion("status", [