fix(betterlist): skip non-numeric indices to prevent ValueError on language page save

[ManthanNimodiya]

Fixes internetarchive/openlibrary#11981

What changed

betterlist.setdefault() in infogami/core/helpers.py called int(index)
unconditionally. When the repetition widget in default_edit.html submits
unprocessed template placeholder rows (e.g. translated_names#[row_translated_names]),
unflatten() passes the placeholder string as the index, causing:

ValueError: invalid literal for int() with base 10: '[row_translated_names]'

This crash made language page editing completely broken via the UI for superusers.

Fix

Wrapped int(index) in a try/except (ValueError, TypeError) block.
Non-numeric indices are silently skipped (return None) — they are always invalid
in a list context and represent unrendered template rows that should be ignored.
All valid numeric indices continue to work exactly as before.

Tests

Added 3 doctests to betterlist.setdefault() covering:

• normal numeric index (existing behaviour)
• placeholder string '[row_translated_names]' → returns None
• list remains unchanged after skipped index

All 15 existing doctests in infogami/core/helpers.py pass.