Fix Mod-Tap combo regression (#20669)

* Add keyevent for combo keyrecord

* Fix formatting

* Update quantum/process_keycode/process_combo.c

Co-authored-by: Sergey Vlasov <sigprof@gmail.com>

* Add combo unit-tests and hot-fix process_record_tap_hint

...as this function tries to lookup the combo keys passed in. This will
be refactored in a later pr.

---------

Co-authored-by: Sergey Vlasov <sigprof@gmail.com>
Co-authored-by: Stefan Kerkmann <karlk90@pm.me>

quantum/process_keycode/process_combo.c

@@ -335,6 +335,7 @@ void apply_combo(uint16_t combo_index, combo_t *combo) {
             // this in the end executes the combo when the key_buffer is dumped.
             record->keycode    = combo->keycode;
             record->event.type = COMBO_EVENT;
+            record->event.key  = MAKE_KEYPOS(0, 0);
 
             qrecord->combo_index = combo_index;
             ACTIVATE_COMBO(combo);