server: fix Windows CI flake in test_completion_unified

[ServeurpersoCom]

Overview

The four requests only match the expected success table when they enter the shared KV pool together. On the Windows runner they are admitted 30 to 70 ms apart, the short one still holds its cells when the long ones reach their limit, and the overflow aborts every slot at once, so a request the table marks as successful comes back with the context error. It now passes on that error too, and on nothing else.

Additional information

Two parameter sets account for 20 failing jobs over the past week, [90, 90, 40, 90] and [90, 90, 40, 75], same shape in both: admissions at 0.504 / 0.504 / 0.533 / 0.534 holding 90 + 90 + 40 + 39 = 259 cells at the overflow, and 0.501 / 0.501 / 0.538 / 0.569 holding 92 + 92 + 42 + 34 = 260. The table and the predicted_n check are untouched, a truncated generation or any other error still fails. The tolerance can go once the TODO in server-context.cpp lands and an overflow terminates only the largest sequence.

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: YES

[ServeurpersoCom]

I sent my agent through every red Server run of the past week to sort the Windows flakes by root cause, here is what it found :

cause                              jobs (7d)  share of router  status

download_finished command lost        42           80%         PR #28747
behind the ANSI reset

DELETE 600s, exit command              5           10%         PR #28755
never sent to the child

DELETE 600s, exit acknowledged         5           10%         PR #28762
but EOF never reached

test_completion_unified               20            -          PR #28759 <- this PR
returning 500

build / infra, misc                   22            -          out of scope

#28747
#28755
#28762