✨ Honor enclosing demands on fields and strings when writing (#447)

[MiWeiss]

Closes #447

What this PR does

Adds an explicit, always-honored enclosing demand to field and string values, and uses it to make month macros (and other bibtex string references) survive writing with the default stack.

1. Field.enclosing / String.enclosing (new model attribute)

• Allowed values: '{', '"', 'no-enclosing', or None (default = writer middleware decides).
• AddEnclosingMiddleware honors a non-None demand with precedence over reuse_previous_enclosing, the int-rule, and default_enclosing.
• Assigning a new value resets the demand to None, so a stale demand can never be applied to a changed value (and the demand is naturally "consumed" once the enclosed value is written).

2. Month middlewares set the demand (the actual #447 use-case)

MonthAbbreviationMiddleware and MonthIntMiddleware now demand no-enclosing for the macros/ints they produce. With the default unparse stack:

library = bibtexparser.parse_string("@article{a, month = {1}}")
bibtexparser.write_string(library, prepend_middleware=[MonthAbbreviationMiddleware()])
# now contains `month = jan` (string reference), previously `month = {jan}` (literal)

MonthLongStringMiddleware deliberately sets no demand (full month names are literals).

3. Parse→write roundtrip no longer corrupts references (latent bug)

Previously, a plain parse_string → write_string roundtrip turned month = jan into month = {jan} and pages = intro # outro into {intro # outro} — silently changing semantics. RemoveEnclosingMiddleware now sets the no-enclosing demand on unenclosed, non-numeric values. Bare integers (year = 2019) remain governed by enclose_integers, as before.

Note: This changes default write output for unenclosed non-numeric inputs (verbatim instead of brace-normalized). I'd argue the old behavior was a bug since it changed semantics, but it's in a separate commit (7e37177) for easy discussion/removal.

Drive-by fixes

• AddEnclosingMiddleware._enclose no longer crashes on int values when enclose_integers=False (value.isdigit() → AttributeError).
• Month middlewares' unchanged-month paths returned the Field object instead of its value (assigning the field to itself as field.value).

Design discussion

Alternatives considered (marker str subclass, parser-metadata convention, month special-casing in the writer) — the explicit attribute was chosen because it is discoverable, expresses any enclosing preference (not just "none"), is robust against field-key renames/duplicates, and fails safe via the value-setter reset.

🤖 Generated with Claude Code

[MiWeiss]

reviewed