test: stabilize notebook queue concurrency gate

[Ishaan Gangwani (ishaan1124)]

Summary

• replace fixed 20 ms sleeps with session-workspace release sentinels
• observe first-running, second-queued, then second-running through the public status route
• bound both Python waits and the scenario deadline, with guaranteed release and kernel cleanup

Root cause

Kernel execution durably records the submitted cell before joining KernelQueue. Under full-suite load, the second HTTP request had not completed that durable admission within 20 ms, so queue_depth 0 was valid even though serialization remained correct.

Validation

• pre-fix load reproduced 3 failures in 12 fresh processes
• final Linux Bun 1.3.14 stress: 12 of 12 pass
• post-fix load: 24 of 24 pass
• full notebook route file: 31 pass, 0 fail
• backend and root typechecks, Prettier, diff check, and staged secret scan pass
• independent review: no P0 or P1 blocker

Fixes the sole failure in exact-main CI run 31780153018.

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Project	Deployment	Actions	Updated (UTC)
openscience	Ready	Preview	Aug 14, 2026 8:06am