fix: to_json no longer mutates caller's ops

[berendkleinhaneveld]

Summary

patchdiff.to_json(ops) was mutating the caller's ops. to_str_paths did a shallow ops.copy(), so the inner dicts were still shared. Overwriting op[\"path\"] with str(op[\"path\"]) then replaced the Pointer in the caller's dicts with strings, which broke subsequent apply() calls. Fix by copying each dict ({**op, \"path\": str(op[\"path\"])}) instead of just the outer list.

Reproducer on master

uv run python -c "
from patchdiff import diff, apply, to_json
a = {'x': [1, 2, 3]}
b = {'x': [1, 9, 3]}
ops, _ = diff(a, b)
to_json(ops)
print(apply(a, ops))
"

On master this raises:

Traceback (most recent call last):
  File "<string>", line 7, in <module>
    print(apply(a, ops))
  File "patchdiff/apply.py", line 48, in apply
    return iapply(deepcopy(obj), patches)
  File "patchdiff/apply.py", line 14, in iapply
    parent, key, _ = ptr.evaluate(obj)
AttributeError: 'str' object has no attribute 'evaluate'

On this branch it prints {'x': [1, 9, 3]}.

New test

• tests/test_serialize.py::test_to_json_does_not_mutate_ops — calls to_json(ops), discards the result, then asserts apply(a, ops) == b. Fails on master with the traceback above; passes on this branch.

Existing tests

• tests/test_serialize.py::test_to_json still passes (sanity check on the JSON output).
• Full uv run pytest -x -q is green (269 passed).

Test plan

• uv run pytest tests/test_serialize.py -v
• uv run pytest -x -q
• uv run ruff check patchdiff/serialize.py tests/test_serialize.py
• uv run ruff format --check patchdiff/serialize.py tests/test_serialize.py

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