Call IsolateObserver::TeardownFinished() only after an isolate's API object has been destroyed #10249

Job	Run time
lint / bazel	7m 5s
lint / bazel	7m 5s
fixup / block-fixup	7s
fixup / block-fixup	7s
labels / InternalPRRequired	1s
labels / InternalPRRequired	1s
test (linux, ubuntu-20.04, -asan) / bazel	12m 47s
test (linux, ubuntu-20.04, -asan) / bazel	12m 47s
test (linux, ubuntu-20.04) / bazel	10m 1s
test (linux, ubuntu-20.04) / bazel	10m 1s
test (linux, ubuntu-20.04, -debug) / bazel	9m 41s
test (linux, ubuntu-20.04, -debug) / bazel	9m 41s
test (windows, windows-2022) / bazel	23m 45s
test (windows, windows-2022) / bazel	16m 58s
test (macOS, macos-15) / bazel	9m 31s
test (macOS, macos-15) / bazel	9m 31s
	2h 19m 9s

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