async: free worker ctx->last_diag before free(ctx) so throws don't leak #149

Job	Run time
ubuntu-24.04	1m 25s
ubuntu-24.04-arm	1m 29s
macos-14	55s
windows-2022	49s
perf-gate (linux)	23s
external-test-suite (clojure-test-suite)	1m 8s
	6m 9s

Provide feedback