Support "upsert" behavior when applying patches

[ohkinozomu]

Currently, applying a patch with an operation like {insert} fails when the target key does not exist in the input document. It would be useful if cels could optionally perform an upsert-like behavior — that is, automatically create missing keys when applying certain operations.

Reproduction

from cels import patch_document

patch = """\
list {insert}: c
"""

case1 = """\
foo:
  bar: 1
  baz: 2
list:
- a
- b
"""

case2 = """\
foo:
  bar: 1
  baz: 2
"""

def run_case(name, input_yaml):
    print(f"=== {name} ===")
    output = patch_document(
        input_format="yaml",
        input_text=input_yaml,
        patch_format="yaml",
        patch_text=patch,
        output_format="yaml",
    )
    print(output)

run_case("case1 (list exists)", case1)
run_case("case2 (list missing)", case2)

Result:

=== case1 (list exists) ===
foo:
  bar: 1
  baz: 2
list:
- a
- b
- c

=== case2 (list missing) ===
Traceback (most recent call last):
  File "/Users/ookinozomu/project6/tmp/python/main.py", line 34, in <module>
    run_case("case2 (list missing)", case2)
  File "/Users/ookinozomu/project6/tmp/python/main.py", line 24, in run_case
    output = patch_document(
             ^^^^^^^^^^^^^^^
  File "/Users/ookinozomu/project6/tmp/python/.venv/lib/python3.12/site-packages/cels/services/patch_document.py", line 105, in patch_document
    output_dict = patch_dictionary(
                  ^^^^^^^^^^^^^^^^^
  File "/Users/ookinozomu/project6/tmp/python/.venv/lib/python3.12/site-packages/cels/services/patch_dictionary.py", line 27, in patch_dictionary
    return patch_dictionary_rec(
           ^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ookinozomu/project6/tmp/python/.venv/lib/python3.12/site-packages/cels/services/patch_dictionary.py", line 69, in patch_dictionary_rec
    change.apply(output_dict, key, patch, path, root_input_dict)
  File "/Users/ookinozomu/project6/tmp/python/.venv/lib/python3.12/site-packages/cels/models/change.py", line 122, in apply
    action(
  File "/Users/ookinozomu/project6/tmp/python/.venv/lib/python3.12/site-packages/cels/models/actions/__init__.py", line 17, in wrapped_func
    return action_func(
           ^^^^^^^^^^^^
  File "/Users/ookinozomu/project6/tmp/python/.venv/lib/python3.12/site-packages/cels/models/actions/action_insert.py", line 14, in action_insert
    safe_extend(container[index], None, [change_value])
                ~~~~~~~~~^^^^^^^
KeyError: 'list'

Expected behavior

It would be helpful if cels could create the missing key when applying a patch, for example:

# patch
list {insert}: c

If list does not exist, it would automatically be created as a list and then have the element appended.

Result:

foo:
  bar: 1
  baz: 2
list:
- c

Possible approaches

1. Make upsert the default behavior
   When an {insert} (or similar) operation targets a missing key, cels would automatically create it with a suitable type (list, mapping, etc.).

2. Introduce a new {upsert} operator
   A new operator could combine the semantics of {add} and {insert} — creating the key if missing, or inserting into it if it already exists.