Fix AttributeError on an out-of-order table under an indented header

[cig13zs]

Fixes #661. Table.append copies the parent's indent onto the item it just stored, but when that key is split out of order the lookup returns an OutOfOrderTableProxy, which has no trivia. The fix skips the proxy, since its parts already keep their own indentation. #263 was the same crash one level up and only stopped happening because the outermost super table has no indent.

[Wkhanex]

AI-assisted validation at d0484992a8bd on Python 3.12.14: I expanded the indentation case to spaces, a tab, and mixed space/tab indentation at depths 4–6, for both a table and an array of tables. Each input also had an intervening unrelated table and a value added through the reopened parent. All 18 combinations parsed and preserved the original text before mutation, and their serialized data after mutation matched tomllib. The 23 focused out-of-order tests passed.

The mutation check matters here because skipping proxy trivia must leave its component tables usable after the initial parse, as well as avoid the AttributeError. I did not find a discrepancy in those cases.