Use `.is_debug_mode()` for maybe-crash-handling

Such that the default is `None` and in the case where the caller *does
not* set the `pdb` arg to an explicit `bool` we instead determine it via
the output from `._state.is_debug_mode()` allowing for more "nonchalant"
usage throughout a (test) code base which passes the `debug_mode: bool`
as runtime config; allows delegation to the per-actor proc-global state.

tractor/devx/_debug.py

@@ -3280,7 +3280,7 @@ def open_crash_handler(
 
 @cm
 def maybe_open_crash_handler(
-    pdb: bool = False,
+    pdb: bool|None = None,
     tb_hide: bool = True,
 
     **kwargs,
@@ -3291,7 +3291,11 @@ def maybe_open_crash_handler(
 
     Normally this is used with CLI endpoints such that if the --pdb
     flag is passed the pdb REPL is engaed on any crashes B)
+
     '''
+    if pdb is None:
+        pdb: bool = _state.is_debug_mode()
+
     __tracebackhide__: bool = tb_hide
 
     rtctx = nullcontext(