server: fix heap-buffer-overflow from negative n_discard (CVE-2026-21869) (#22267)

server: fix heap-buffer-overflow from negative n_discard (CVE-2026-21869) (#22267)

* server: clamp n_discard to non-negative at JSON parse boundary (CVE-2026-21869)

A negative n_discard from client JSON causes heap-buffer-overflow in
update_slots() context-shift loop (CWE-787, CVSS 8.8). Clamp to 0 at
ingress; n_discard=0 already triggers auto-discard (n_left/2).

Ref: GHSA-8947-pfff-2f3c

* cont : cleaner

* cont : cleanerer

* cont : cleanest

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Co-authored-by: Georgi Gerganov <redacted>