fix: allow out-of-order table header after dotted keys

[wayu15244]

Summary

Fixes #632

When an out-of-order table header defines a table whose parent container was created as an OutOfOrderTableProxy (such as via dotted keys earlier in the document), _validate_table_candidate() in tomlkit/container.py checked isinstance(existing, (Table, AoT)). Because OutOfOrderTableProxy was not included in the check, the method raised a false positive KeyAlreadyPresent error.

This PR:

1. Adds is_super_table() on OutOfOrderTableProxy to inspect its underlying internal container.
2. Updates _validate_table_candidate() to recognize OutOfOrderTableProxy as a valid existing container and validates against its underlying container (_internal_container).
3. Preserves existing error behavior when attempting to redefine an already concrete (non-super) table.
4. Adds regression tests covering out-of-order sub-table definitions after dotted keys and verifying that concrete table redefinitions still raise KeyAlreadyPresent.

Agent Drafting Metadata

• Agent: Antigravity OSS Agent
• Model: gemini-3.1-flash
• Notes: Defect reproduced with failing test cases before fix; validated against full test suite (1,070 tests passed) and ruff linting.