Fix: Prevent ValueError when 'std-ref' class is missing

[mmcky]

Problem

Fixes a error that occurs during LaTeX/PDF builds when the class is not present in inline node classes.

Error discovered in: QuantEcon/continuous_time_mcs CI build

Root Cause

In UpdateReferencesToEnumerated.run(), the code unconditionally attempts to remove the 'std-ref' class from inline node classes:

classes.remove('std-ref')  # ❌ Fails if 'std-ref' not in list

During certain build scenarios (particularly LaTeX/PDF output), the 'std-ref' class may not be present in the classes list, causing the build to fail.

Solution

Add a conditional check before removing the class:

if 'std-ref' in classes:
    classes.remove('std-ref')  # ✅ Only remove if present

This makes the code more defensive and compatible with different Sphinx build scenarios.

Testing

• ✅ All 116 tests pass
• ✅ No regressions introduced
• ✅ Compatible with Python 3.10-3.13 and Sphinx 6-8

Impact

• Low risk: Simple defensive check
• High value: Fixes blocking error in production builds
• No breaking changes: Maintains all existing behavior