Add maximum size limit for postponed body parsing (#88175)
### What?
Adds a configurable `experimental.maxPostponedStateSize` limit for PPR
postponed state body parsing to prevent OOM/DoS attacks.
### Why?
The postponed state body was read entirely without size limits, creating
a potential denial-of-service vector through unbounded memory
allocation.
### How?
Enforces a 10 MB default limit (configurable via next.config.js) with
byte counting during body parsing. Returns HTTP 413 when exceeded with a
helpful error message directing users to increase the limit if needed.
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