gh-151892: Fix crash unpickling interpreters.call() result

[tonghuaroot]

When Interpreter.call() returns a value, it is preserved across the cross-interpreter session and reconstructed (unpickled) in the calling interpreter inside _PyXI_Exit() (Python/crossinterp.c). If reconstruction raises, the live exception was left set while the function applied its _PyXI_ERR_PRESERVE_FAILURE override, so the subsequent assert(!_PyErr_Occurred(tstate)) aborted on debug builds (and the stale exception was clobbered on release builds).

The reconstruction-failure branch now consumes the live exception and propagates it as the failure, mirroring how the active-session _pop_preserved path already routes errors through capture_session_error. call() now raises concurrent.interpreters.NotShareableError (with the original exception reachable as the cause) instead of crashing; a NotShareableError raised during reconstruction is propagated the same way.

A regression test is added in Lib/test/test_interpreters/test_api.py covering caller-side result-unpickling failures (a plain exception and a NotShareableError); it aborts without the fix and passes with it. test_interpreters / test__interpreters pass and the new test is refleak-clean under -R 3:3.

The affected machinery (_PyXI_Preserve / _finish_preserved, xi_error_*) postdates the 3.14.0b1 freeze, so this targets main only — no backport is needed.

A separate, distinct crossinterp NULL dereference on the queue/channel receive path is tracked in #151862.

• Issue: concurrent.interpreters: Interpreter.call() crashes when the result fails to unpickle in the caller #151892