Fix a potential memory leak in PersistentTaskGroup

[achimnol]

• It no longer stores the unhandled exceptions from subtasks, as they will be kept for the whole lifespan of the PersistentTaskGroup.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.57 :tada:

Comparison is base (540daf2) 74.31% compared to head (0616890) 74.88%.

:exclamation: Current head 0616890 differs from pull request most recent head eb8d186. Consider uploading reports for the commit eb8d186 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #54 +/- ## ========================================== + Coverage 74.31% 74.88% +0.57% ========================================== Files 17 17 Lines 1347 1322 -25 ========================================== - Hits 1001 990 -11 + Misses 346 332 -14 ``` | [Impacted Files](https://codecov.io/gh/achimnol/aiotools/pull/54?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Joongi+Kim) | Coverage Δ | | |---|---|---| | [src/aiotools/server.py](https://codecov.io/gh/achimnol/aiotools/pull/54?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Joongi+Kim#diff-c3JjL2Fpb3Rvb2xzL3NlcnZlci5weQ==) | `48.98% <ø> (ø)` | | | [src/aiotools/func.py](https://codecov.io/gh/achimnol/aiotools/pull/54?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Joongi+Kim#diff-c3JjL2Fpb3Rvb2xzL2Z1bmMucHk=) | `98.03% <100.00%> (+0.03%)` | :arrow_up: | | [src/aiotools/taskgroup/\_\_init\_\_.py](https://codecov.io/gh/achimnol/aiotools/pull/54?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Joongi+Kim#diff-c3JjL2Fpb3Rvb2xzL3Rhc2tncm91cC9fX2luaXRfXy5weQ==) | `100.00% <100.00%> (ø)` | | | [src/aiotools/taskgroup/persistent.py](https://codecov.io/gh/achimnol/aiotools/pull/54?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Joongi+Kim#diff-c3JjL2Fpb3Rvb2xzL3Rhc2tncm91cC9wZXJzaXN0ZW50LnB5) | `90.53% <100.00%> (+3.16%)` | :arrow_up: | | [src/aiotools/taskgroup/persistent\_compat.py](https://codecov.io/gh/achimnol/aiotools/pull/54?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Joongi+Kim#diff-c3JjL2Fpb3Rvb2xzL3Rhc2tncm91cC9wZXJzaXN0ZW50X2NvbXBhdC5weQ==) | `85.39% <100.00%> (+2.58%)` | :arrow_up: | | [src/aiotools/taskgroup/types.py](https://codecov.io/gh/achimnol/aiotools/pull/54?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Joongi+Kim#diff-c3JjL2Fpb3Rvb2xzL3Rhc2tncm91cC90eXBlcy5weQ==) | `90.00% <100.00%> (ø)` | |

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