Represent out-of-order table proxies with merged values

[gaoflow]

Fixes #439.

Summary

• make OutOfOrderTableProxy string/repr output use the merged internal container value
• add a regression test for two dotted-key fragments under the same out-of-order proxy

Verification

• uv run --no-project --with pytest --with pyyaml python -m pytest tests/test_toml_document.py::test_repr_out_of_order_table_proxy tests/test_toml_document.py::test_repr tests/test_toml_document.py::test_out_of_order_tables_are_still_dicts -q
• uv run --no-project --with pytest --with pyyaml python -m pytest -q
• uv run --no-project --with ruff ruff check tomlkit/container.py tests/test_toml_document.py
• uv run --no-project --with ruff ruff format --check tomlkit/container.py tests/test_toml_document.py
• python -m compileall tomlkit/container.py tests/test_toml_document.py
• git diff --check