fix(api): populate lhsrawalt in nvim_get_keymap response

Problem:
The `nvim_get_keymap()` function is missing the `lhsrawalt` field in its response for mappings with an alternate key representation. This makes its return value inconsistent with its documented `maparg()`-like structure and its formal type definition.

Solution:
Corrects the `keymap_array` function to pass the alternate mapping keys (`current_maphash->m_alt->m_keys`) to `mapblock_fill_dict`. The argument responsible for this was previously hardcoded to `NULL`.

For example, for a mapping of `<C-x>`, the API will now correctly return both `lhsraw` (`<80><fc>^DX`) and `lhsrawalt` (the alternate form, e.g., `^X`).

(cherry picked from commit d523750de0)

src/nvim/mapping.c

@@ -2894,7 +2894,10 @@ ArrayOf(Dict) keymap_array(String mode, buf_T *buf, Arena *arena)
       }
       // Check for correct mode
       if (int_mode & current_maphash->m_mode) {
-        kvi_push(mappings, DICT_OBJ(mapblock_fill_dict(current_maphash, NULL, buffer_value,
+        kvi_push(mappings, DICT_OBJ(mapblock_fill_dict(current_maphash,
+                                                       current_maphash->m_alt
+                                                       ? current_maphash->m_alt->m_keys : NULL,
+                                                       buffer_value,
                                                        is_abbrev, false, arena)));
       }
     }