`Array.add_line()` doesn't serialize as documented (or expected) and can even produce syntactically invalid TOML

Start with Array.add_line and follow how as_string serializes single-line arrays, then reproduce the commented example and verify the output through tomlkit.loads. Done means adding a line with a comment produces valid TOML and the resulting array serialization consistently reflects multiline behavior; add regression coverage for both direct and document serialization.

Update: This seems to be part of a larger problem; see my comment below for additional context.

If an Array isn't multiline, then calling its .add_line method doesn't add a trailing line separator. When the added line includes a comment, this causes .as_string and similar to produce invalid TOML:

>>> import tomlkit
>>> array = tomlkit.array()
>>> array.add_line("foo", comment="bar")
>>> print(array.as_string())
[
    "foo", # bar]
>>> doc = tomlkit.document()
>>> doc.add("array", array)
{'array': ['foo']}
>>> print(doc.as_string())
array = [
    "foo", # bar]

>>> tomlkit.loads(doc.as_string())
Traceback (most recent call last):
  File "<python-input-7>", line 1, in <module>
    tomlkit.loads(doc.as_string())
    ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/home/five35/.local/share/hatch/env/virtual/python-scripts/S8f9twao/python-scripts/lib/python3.14/site-packages/tomlkit/api.py", line 52, in loads
    return parse(string)
  File "/home/five35/.local/share/hatch/env/virtual/python-scripts/S8f9twao/python-scripts/lib/python3.14/site-packages/tomlkit/api.py", line 99, in parse
    return Parser(string).parse()
           ~~~~~~~~~~~~~~~~~~~~^^
  File "/home/five35/.local/share/hatch/env/virtual/python-scripts/S8f9twao/python-scripts/lib/python3.14/site-packages/tomlkit/parser.py", line 139, in parse
    item = self._parse_item()
  File "/home/five35/.local/share/hatch/env/virtual/python-scripts/S8f9twao/python-scripts/lib/python3.14/site-packages/tomlkit/parser.py", line 238, in _parse_item
    return self._parse_key_value(True)
           ~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/home/five35/.local/share/hatch/env/virtual/python-scripts/S8f9twao/python-scripts/lib/python3.14/site-packages/tomlkit/parser.py", line 331, in _parse_key_value
    val = self._parse_value()
  File "/home/five35/.local/share/hatch/env/virtual/python-scripts/S8f9twao/python-scripts/lib/python3.14/site-packages/tomlkit/parser.py", line 435, in _parse_value
    return self._parse_array()
           ~~~~~~~~~~~~~~~~~^^
  File "/home/five35/.local/share/hatch/env/virtual/python-scripts/S8f9twao/python-scripts/lib/python3.14/site-packages/tomlkit/parser.py", line 612, in _parse_array
    raise self.parse_error(UnexpectedCharError, self._current)
tomlkit.exceptions.UnexpectedCharError: Unexpected character: '\x00' at line 2 col 0

I think the simplest fix might be to make calling .add_line implicitly change single-line arrays to multiline. If nothing else, it doesn't make semantic sense to "add a line" to something which can't contain multiple lines, so hopefully that behavior wouldn't be surprising to anyone.