[Abhayanthk]

What type of PR is this?
/kind bug

What this PR does / why we need it:
When a grace period is set, a slot freed by a finished request should be held for a same-session follow-up until the grace period ends. While testing go test -race on main, I saw TestSessionBoostQueue_GracePeriod_NoBoostArrives fail with "Dequeue happened too early (before grace period)".

The cause is a small timing window. Release first lowers the inflight count and then sends the release signal. The arrival path checks the release signal first and then reads the inflight count. If a release lands between those two reads, the arrival path sees no pending release but does see the free slot, so it admits a waiting request right away and the grace hold is skipped.

The fix makes both sides read the same state. Release now lowers the count and sends the signal under a new releaseMu. The arrival path reads the release signal and the inflight count together under the same lock, and then admits only above the count it read. A slot freed after that read stays held for the grace path.

I added a test case that frees a slot in the middle of the arrival handling. It fails on main and passes with the fix.

Which issue(s) this PR fixes:
None

Bug evidence (required for bug-related PRs):

• On main, TestSessionBoostQueue_GracePeriod_NoBoostArrives failed once in 3 full -race runs of the make test packages.
• With a 20 ms sleep added in Release between the decrement and the signal, the same test fails 20 out of 20 times on main and 0 out of 20 with the fix.
• The new table case fails 20 out of 20 times on main and passes 20 out of 20 with the fix.

Special notes for your reviewer:
I used AI tools while working on this and verified everything myself.

Does this PR introduce a user-facing change?:

Fixed a race in the router session boost queue where a freed slot could be given to a waiting request before the grace period ended.

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[Abhayanthk]

/cc @YaoZengzeng @kube-gopher @LiZhenCheng9527
PTAL when you have time.