fix: prevent Task.get deadlocks from threadpool starvation

[Kha]

This PR prevents deadlocks from non-cyclical task waits that may otherwise occur during parallel elaboration with small threadpool sizes.

[Kha]

!bench

[leanprover-bot]

Here are the benchmark results for commit ea69d56.
There were significant changes against commit 9b74c07:

  Benchmark                   Metric          Change
  ============================================================
+ big_do                      branch-misses    -1.6% (-22.3 σ)
+ big_omega.lean              task-clock       -2.5% (-15.6 σ)
+ big_omega.lean              wall-clock       -2.5% (-15.1 σ)
+ bv_decide_inequality.lean   branch-misses    -1.2% (-12.2 σ)
+ bv_decide_mul               branch-misses    -2.4% (-46.0 σ)
+ const_fold                  task-clock       -3.8% (-13.7 σ)
+ const_fold                  wall-clock       -3.9% (-14.2 σ)
+ language server startup     branch-misses    -2.2% (-10.1 σ)
+ language server startup     maxrss           -2.1% (-20.4 σ)
- nat_repr                    task-clock        4.7%  (12.1 σ)
- nat_repr                    wall-clock        4.5%  (14.0 σ)
+ stdlib                      type checking    -1.1% (-16.3 σ)

[leanprover-community-bot]

Mathlib CI status (docs):

• ✅ Mathlib branch lean-pr-testing-6758 has successfully built against this PR. (2025-01-23 18:08:47) View Log