Fix regression parsing out-of-order tables split by another table

[faresrafat3]

Fixes #571.

tomlkit 0.15.1 rejects a valid document that tomllib and tomlkit
0.13.3/0.14.0/0.15.0 all accept:

import tomlkit
tomlkit.loads("""\
[tool.ruff]
[tool.ruff.lint.a]
[tool.ruff.lint]
[[tool.poetry.source]]
[tool.ruff.lint.b]
""")
# tomlkit.exceptions.ParseError: Key "lint" already exists. at line 5 col 0

All three ingredients are needed: an out-of-order child, its concrete parent
declared afterwards, and a later sibling child separated by an unrelated table.

Root cause

When a key is defined out of order, Container._raw_append records several
body positions for it and Container.item() then returns an
OutOfOrderTableProxy instead of the Table:

if isinstance(idx, tuple):
    # The item we are getting is an out of order table
    return OutOfOrderTableProxy(self, idx)

_validate_table_candidate compared the candidate against that proxy using

if isinstance(existing, (Table, AoT)) != isinstance(v, (Table, AoT)):
    raise KeyAlreadyPresent(k)

isinstance(proxy, (Table, AoT)) is False — the proxy is a separate class —
so a valid table-vs-table extension looked like a table-vs-scalar conflict and
raised KeyAlreadyPresent. Traced at the failure point:

[1] current super=False body=[('lint', 'Table'), ('lint', 'Table')]
    candidate super=True body=[('lint', 'Table')]
    CONFLICT key=lint existing=OutOfOrderTableProxy v=Table

Change

Skip the type comparison when the existing entry is an out-of-order proxy; its
fragments are already validated by the proxy. One condition, three lines of
comment.

Verification

The reported document now parses and round-trips byte-identically, and its
unwrap() matches tomllib exactly:

>>> tomlkit.loads(src).unwrap() == tomllib.loads(src)
True
>>> tomlkit.loads(src).as_string() == src
True

Genuine duplicates are still rejected — checked explicitly, each raising:

input	result
[a]\n[a]\n	ParseError
[a]\nb=1\n[a]\n	ParseError
[a]\n[a.b]\n[a.b]\n	KeyAlreadyPresent
a=1\n[a]\n	ParseError
[a]\n[[a]]\n	ParseError
[a]\nb.c=1\n[a.b]\n	TOMLKitError (dotted redefinition)

Two regression tests added to tests/test_toml_document.py (the issue's
document, and a deeper variant without the array-of-tables). Full suite:
390 passed (388 before).

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[dimbleby]

Explain why not #572