Fix scitoken_status_free: missing C-linkage symbol and status type confusion

[djw8605]

Problem

Three related bugs in the async-status C API:

1. The C symbol doesn't exist. scitokens.h declares void scitoken_status_free(SciTokenStatus *status) inside extern "C", but scitokens.cpp defines scitoken_status_free(SciTokenStatus status) — a different parameter type. C++ treats that as a separate overload with C++ linkage, so the library only exports the mangled _Z20scitoken_status_freePv. Any C consumer using the public header gets an undefined symbol at link time. On Linux it's worse: the mangled name doesn't match the scitoken* glob in configs/export-symbols, so the function isn't exported in any form. (Verified with nm before/after.)

2. Type confusion on free. The implementation deletes the handle as scitokens::AsyncStatus*, but statuses returned by scitoken_deserialize_start are SciTokenAsyncStatus* — an unrelated type holding two unique_ptrs. Deleting through the wrong type runs ~AsyncStatus over unrelated memory (its std::unique_lock members get destroyed reading garbage — potential unlock of a wild mutex pointer) and leaks the real members.

3. Accessor type confusion for enforcer statuses. scitoken_status_get_timeout_val / _get_read_fd_set / etc. cast the handle to SciTokenAsyncStatus*, but enforcer_generate_acls_start returned a raw AsyncStatus* — so using the select()-loop accessors with the enforcer async API read garbage.

Fix

• Match the declared signature; also null the caller's handle and make null input a no-op.
• Wrap enforcer async statuses in SciTokenAsyncStatus (start) and unwrap on continue, so every SciTokenStatus handle exposed through the C API is the same concrete type. This simultaneously fixes the free path and the scitoken_status_get_* accessors for enforcer statuses.
• Reject a null status_out in scitoken_deserialize_start instead of dereferencing it.

Testing

• New regression test frees an async status mid-flight (and a null handle); the test calling the function at all verifies the C-linkage symbol resolves — it did not before this change.
• nm libSciTokens.dylib now shows T _scitoken_status_free (unmangled).
• ctest unit, env_config, and monitoring suites pass.

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