mruby-task: raise exception when task.pass is called from C function

when task.pass is called from within a C function (such as Module.new's
block evaluation), attempting to yield would cause a segfault because C
functions lack valid bytecode program counters (see #6642).

this commit adds C function boundary detection to task.pass, raising a
runtime error when cci > 0 (indicating execution is inside a C function).
this matches fiber's behavior and provides a clear error message instead of
a cryptic segfault.

unlike the previous commit which allowed sleep to fall back to blocking
sleep, task.pass raises an exception because its sole purpose is cooperative
yielding - there is no sensible blocking fallback behavior.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-task/src/task.c

@@ -770,6 +770,14 @@ mrb_task_s_pass(mrb_state *mrb, mrb_value self)
     task_run_one_iteration(mrb);
   }
   else {
+    /* Check for C function boundary - cannot yield from C function */
+    mrb_callinfo *ci;
+    for (ci = mrb->c->ci; ci >= mrb->c->cibase; ci--) {
+      if (ci->cci > 0) {
+        mrb_raise(mrb, E_RUNTIME_ERROR, "can't pass across C function boundary");
+      }
+    }
+
     /* In task context - trigger context switch */
     switching_ = TRUE;
   }