reject a comment containing line breaks in Item.comment and add_line

[netliomax25-code]

Summary

1. Item.comment() and Array.add_line(comment=...) accepted a comment containing an embedded newline and rendered everything after the first line without a #, so the output no longer re-parses (tomlkit and tomllib both reject it).
2. Per review these two APIs are single-line only, so a comment containing a line break now raises ValueError up front instead of being reformatted; the item is left unmodified when it raises.
3. Single-line comment behavior is unchanged. Tests cover both rejection paths; full suite passes.

Agent Drafting Metadata

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[netliomax25-code]

any update?

[frostming]

They are different thing, add_line() and item.comment() are expected to accept single line comment ONLY. In case of string containing line breaks, you should reject it.

[netliomax25-code]

Makes sense, reworked it that way:

1. Item.comment() and add_line(comment=...) now raise ValueError when the comment contains a line break, instead of trying to reformat it.
2. Single-line behavior is unchanged; tests updated to check the rejection and the full suite still passes.

Updated the title and description to match.