Resolve calc() division by zero to infinity instead of crashing - #2819

The bug

A division by zero inside a calc() expression raises ZeroDivisionError and aborts the entire render:

from weasyprint import HTML
HTML(string='<div style="width: calc(100px / 0)">x</div>').write_pdf()
# Traceback ...
#   File ".../weasyprint/css/__init__.py", line 724, in _resolve_calc_product
#     value /= calc.value
# ZeroDivisionError: division by zero

It also fires when the divisor merely evaluates to zero, e.g. calc(100px / (1 - 1)).

Why the expected behaviour is unambiguous

CSS Values and Units 4 §10.10 requires a division by zero to resolve to +∞, −∞, or NaN following IEEE-754, not to be an error: a positive numerator gives +∞, a negative one −∞, and 0 / 0 gives NaN. WeasyPrint already supports infinite and NaN values in calc() (calc(infinity * 1px), calc(nan * 1px), and the clamp() tests use calc(-infinity * 1px)), so only the division operator was missing the case.

The fix

In _resolve_calc_product, when the divisor is zero, produce ±infinity/NaN (IEEE-754) instead of dividing. The result is identical to the equivalent calc(infinity * …) / calc(nan * …) expression, which then flows through the existing infinity handling (an infinite width renders as inf, a negative-infinite/NaN width clamps to 0, exactly as the keyword forms already do).

Tests

test_calc_division_by_zero asserts the rendered width of each division-by-zero form equals that of its calc(infinity * …) / calc(nan * …) counterpart — covering positive/negative numerators, relative units, a divisor that evaluates to zero, 0 / 0 (NaN), and infinity propagating through a following +/*. All eight cases fail on main with the original ZeroDivisionError and pass with the fix; the full tests/css/test_math.py (439 passed) and test_validation.py stay green, ruff check clean.