fix: do not leave an unreturned table behind when the style is undefined (#1631) - #1637

Document.add_table() assigns table.style = style after _Body.add_table() has already appended the w:tbl element to the body. When the style name is not defined in the document, Table.style raises KeyError — and the half-built table stays in the body. The caller gets an exception and no table object, so it cannot remove the stray table either.

This rolls the table back before re-raising, so a failed call leaves the document exactly as it was:

table = self._body.add_table(rows, cols, self._block_width)
try:
    table.style = style
except KeyError:
    table._tbl.getparent().remove(table._tbl)
    raise
return table

The KeyError still propagates (the caller should still learn the style name was wrong) — only the side effect is undone. This also matches what save()-then-reopen showed before the fix: the stray table was really written to the file, not just kept in memory.

The regression test builds a real document (no mocks), calls add_table() with an undefined style and asserts document.tables stays empty. It is a module-level it_... function because python_functions in pyproject.toml only collects that prefix.

Test results locally (Windows, Python 3.12, pyparsing==3.1.1 — 3.3.3 trips filterwarnings = ["error"]):

• before: 53 failed, 1547 passed, 9 errors
• after: 53 failed, 1548 passed, 9 errors

Same failure set (all pre-existing Windows temp-file/path issues), +1 new passing test. ruff check and ruff format --check are clean.

Fixes #1631.