fix(ibus): keep engine teardown correct when parent destroy fails (#613)

* fix(ibus): keep engine teardown correct when parent destroy fails

`do_destroy` passed `super().do_destroy` to `_handle_engine_destroy`.
PyGObject binds that vfunc to the GType rather than to the instance, so
calling it with no arguments raises:

    TypeError: IBus.Object.destroy() takes exactly 1 argument (0 given)

This fired on every engine teardown. The exception escaped
`_handle_engine_destroy` before its return, so neither
`_active_instance = ...` nor `_focus_event.clear()` in the caller ran,
leaving a destroyed engine registered as active with focus still set --
the two values `_ibus_ping_status` (#523) reads to decide commit
readiness.

Two changes:

- Bind the parent destroy to the instance, falling back to
  `IBus.Object.destroy(self)` only when the no-arg call raises TypeError,
  so this keeps working if PyGObject binds the vfunc correctly later.
- Guard the parent destroy call so no failure there can skip teardown.

Fixes #606

* refactor(ibus): extract parent-destroy workaround into a testable helper

Codecov flagged the new code as uncovered (36% patch, below the 80%
target). The whole `if IBUS_AVAILABLE:` branch of `do_destroy` is
unreachable from the suite, which only ever exercises that method with
IBUS_AVAILABLE False -- the single line it replaced was already
uncovered on main for the same reason.

Move the workaround into a module-level `_invoke_parent_destroy`, the
same pattern `_handle_engine_destroy` already uses to make `do_destroy`
testable, and bind it with functools.partial at the call site.

Behaviour is unchanged. Coverage of the branch goes from 0 to all but
the one-line partial, and three tests now pin the actual contract:
pass-through when the binding is correct, retry with the instance on the
GType-binding TypeError, and no masking of other exceptions.

src/vocalinux/text_injection/ibus_engine.py

@@ -14,6 +14,7 @@ This is the preferred method for Wayland environments as it works universally
 without requiring compositor-specific protocols.
 """
 
+import functools
 import logging
 import os
 import signal
@@ -589,6 +590,23 @@ def restore_xkb_layout(layout: str, variant: str = "", option: str = "") -> bool
     return False
 
 
+def _invoke_parent_destroy(parent_destroy: Callable[[], None], instance: object) -> None:
+    """Call the parent engine destroy, working around PyGObject's GType binding.
+
+    PyGObject binds the ``do_destroy`` vfunc to the GType rather than to the
+    instance, so ``super().do_destroy()`` raises
+    ``TypeError: IBus.Object.destroy() takes exactly 1 argument (0 given)``.
+    Retry with the instance explicitly.
+
+    The TypeError path rather than an unconditional ``IBus.Object.destroy``
+    keeps this working if a future PyGObject binds the vfunc correctly.
+    """
+    try:
+        parent_destroy()
+    except TypeError:
+        IBus.Object.destroy(instance)
+
+
 def _handle_engine_destroy(
     active_instance: Optional["VocalinuxEngine"],
     current_instance: object,
@@ -601,7 +619,13 @@ def _handle_engine_destroy(
         next_active_instance = None
 
     if ibus_available and super_destroy is not None:
-        super_destroy()
+        # The caller assigns our return value to _active_instance and clears the
+        # focus event, so a failing parent destroy must not skip that teardown:
+        # otherwise a destroyed engine stays registered as active with focus set.
+        try:
+            super_destroy()
+        except Exception:
+            logger.exception("Parent engine destroy failed; continuing teardown")
 
     return next_active_instance
 
@@ -866,7 +890,7 @@ class VocalinuxEngine(IBus.Engine if IBUS_AVAILABLE else object):
         logger.debug("VocalinuxEngine instance destroyed")
         super_destroy: Optional[Callable[[], None]] = None
         if IBUS_AVAILABLE:
-            super_destroy = super().do_destroy
+            super_destroy = functools.partial(_invoke_parent_destroy, super().do_destroy, self)
         was_active = VocalinuxEngine._active_instance is self
         VocalinuxEngine._active_instance = _handle_engine_destroy(
             VocalinuxEngine._active_instance,