Expand error release and handler test coverage

tests/err_release_null.c

@@ -0,0 +1,43 @@
+#include "akerror.h"
+#include "err_capture.h"
+#include <string.h>
+
+/*
+ * akerr_release_error(NULL) is an API contract violation the library reports
+ * rather than crashes on: it raises AKERR_NULLPOINTER against the internal
+ * last-ditch context and hands that back, so the caller gets a describable
+ * error instead of a dereferenced NULL. Nothing exercised that path.
+ *
+ * The last-ditch context deliberately lives outside AKERR_ARRAY_ERROR so
+ * reporting this failure cannot consume a pool slot -- which is exactly what
+ * akerr_valid_error_address() reports on, and what this test checks.
+ */
+
+int main(void)
+{
+    akerr_capture_install();
+    /* akerr_init() sets up the last-ditch context's stacktrace cursor. */
+    akerr_init();
+    AKERR_CHECK(akerr_slots_in_use() == 0);
+
+    akerr_ErrorContext *ret = akerr_release_error(NULL);
+
+    AKERR_CHECK(ret != NULL);
+    AKERR_CHECK(ret->status == AKERR_NULLPOINTER);
+    AKERR_CHECK(strstr(ret->message, "NULL context pointer") != NULL);
+
+    /* Reported through FAIL, so the stack trace names the error too. */
+    AKERR_CHECK(strstr(ret->stacktracebuf, "Null Pointer Error") != NULL);
+
+    /* Not a pool slot, and no slot was checked out to report the failure. */
+    AKERR_CHECK(akerr_valid_error_address(ret) == 0);
+    AKERR_CHECK(akerr_slots_in_use() == 0);
+
+    /* The last-ditch context is a singleton: the same one comes back. */
+    akerr_ErrorContext *again = akerr_release_error(NULL);
+    AKERR_CHECK(again == ret);
+    AKERR_CHECK(akerr_slots_in_use() == 0);
+
+    fprintf(stderr, "err_release_null ok\n");
+    return 0;
+}