reject array-of-tables header missing its second closing bracket

[netliomax25-code]

Summary

When a table header is opened with [[ for an array of tables, _parse_table skips the first closing bracket after checking it is ], but then consumes a second character as the matching close bracket without verifying it, behind a leftover # TODO: Verify close bracket. A header opened with [[ and closed with a single ] (or with trailing junk after one ], or that hits EOF) is silently accepted: inputs like [[a]b, [[a], [[a.b]x and [[a]x parse as [[a]], and round-tripping rewrites the malformed header to the valid form, so an out-of-spec document is accepted and its text changed under you. tomllib and the toml-test suite reject all of these. The fix mirrors the existing single-bracket check: confirm the second character is ] (raising UnexpectedCharError, or UnexpectedEofError at end of input) before consuming it. Added regression tests for the missing-bracket forms.

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