🐛 Keep month field enclosing when the value is not transformed

[MiWeiss]

_MonthInterpolator.transform_entry assigns month.value even when the month could not be resolved. The Field.value setter resets enclosing, so the no-enclosing demand on a reference is lost and AddEnclosingMiddleware braces it into a literal.

>>> lib = bibtexparser.parse_string("@article{a, month = foo}", append_middleware=[MonthLongStringMiddleware()])
>>> bibtexparser.write_string(lib)
'@article{a,\n\tmonth = {foo}\n}\n'

Fix: assign only when the value actually changed, comparing type too, since "1" to 1 is a real transformation. _demanded_enclosing and the metadata line stay outside the guard, because MonthAbbreviationMiddleware demands no-enclosing for an already-correct jan.

model.py untouched; the setter's reset is documented behavior.

Tests: 5 cases across all three month middlewares, 3 fail before. Suite 2581 passed, from 2576.

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🤖 Generated with Claude Code

[MiWeiss]

lgtm