fix: infinite loop when RULE_CALLABLE returns zero consumed characters

[gaoflow]

A RULE_CALLABLE that returns zero consumed characters causes an
infinite loop in unicode_to_latex().

Root cause: _apply_rule_callable() passes the consumed count
from the callable directly to _apply_replacement() which does
p.pos += numchars. When numchars == 0, the position never
advances, and the while p.pos < len(s) loop re-fires the same
rule on the same position endlessly.

Fix (+9 lines): Guard in _apply_rule_callable() that raises
ValueError if consumed <= 0, with a descriptive message pointing
to the offending callable and input position. Prevents the hang and
gives actionable feedback.

Reproduction:

from pylatexenc.latexencode import unicode_to_latex
unicode_to_latex('a', [([('a',)], lambda s, pos: (0, 'X'))])
# Before: hangs forever (infinite loop)
# After:  ValueError: RULE_CALLABLE for rule ... returned 0

286/286 tests pass.

This pull request was prepared with the assistance of AI, under my
direction and review.