Harden `Unpacker.init` re-entry cleanup to prevent buffer/context leaks #451

Job	Run time
Run test with Python 3.15t on windows-latest	2m 17s
Run test with Python 3.12 on ubuntu-latest	28s
Run test with Python 3.15 on ubuntu-latest	36s
Run test with Python 3.14 on windows-latest	1m 19s
Run test with Python 3.10 on windows-latest	1m 3s
Run test with Python 3.15t on ubuntu-latest	1m 6s
Run test with Python 3.12 on windows-latest	1m 21s
Run test with Python 3.11 on windows-latest	1m 30s
Run test with Python 3.10 on ubuntu-latest	26s
Run test with Python 3.14t on windows-latest	3m 4s
Run test with Python 3.13 on windows-latest	51s
Run test with Python 3.11 on ubuntu-latest	28s
Run test with Python 3.13 on windows-11-arm	1m 14s
Run test with Python 3.14 on ubuntu-latest	28s
Run test with Python 3.15 on windows-latest	1m 46s
Run test with Python 3.15 on macos-latest	1m 0s
Run test with Python 3.14 on windows-11-arm	1m 6s
Run test with Python 3.14 on macos-latest	35s
Run test with Python 3.12 on macos-latest	28s
Run test with Python 3.14t on macos-latest	1m 24s
Run test with Python 3.11 on windows-11-arm	2m 14s
Run test with Python 3.15t on windows-11-arm	2m 59s
Run test with Python 3.11 on macos-latest	30s
Run test with Python 3.12 on windows-11-arm	1m 4s
Run test with Python 3.13 on macos-latest	22s
Run test with Python 3.14t on windows-11-arm	2m 51s
Run test with Python 3.15 on windows-11-arm	2m 54s
Run test with Python 3.10 on macos-latest	35s
Run test with Python 3.15t on macos-latest	1m 22s
Run test with Python 3.14t on ubuntu-latest	1m 15s
Run test with Python 3.13 on ubuntu-latest	29s
	39m 5s

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