key not found in hash table during check(3, "ThreadedGB")

Nobody has claimed this yet.

I've seen the following happen a couple times recenty. Probably some race condition, maybe a consequence of #3186?

```m2
ThreadedGB.m2:648:1-656:1 error:
-- R = ZZ/101[x,y,z, MonomialOrder=>GLex];
--
-- i2 : I = {x*y*z + z^2, x*y^2 - y^3*z + x*z, y^2};
--
-- i3 : allowableThreads= 4;
--
-- i4 : H = tgb(I);
--
-- i5 : assert (x^2*z == H#(0,1))
-- stdio:7:23:(3): error: key not found in hash table
--
../m2/debugging.m2:18:6:(1):[9]: error: test(s) #3 of package ThreadedGB failed.
```

[full log](https://launchpadlibrarian.net/728328707/buildlog_ubuntu-mantic-arm64.macaulay2_1.23.0.1+git202405051947-0ppa202405051952~ubuntu23.10.1_BUILDING.txt.gz)

Start by reproducing the provided ThreadedGB example from ThreadedGB.m2:648 and inspect the failing check(3, "ThreadedGB") run. Read debugging.m2 around test #3 and compare repeated runs with allowableThreads=4; done means the assertion no longer produces a key-not-found error.

Written by the indexing model from the issue text.