Fix OneOf.options() emitting phantom entries when labels outnumber choices

[rstar327]

Fixes #2869

OneOf.options() used zip_longest(choices, labels, fillvalue="") which produced phantom ('', 'label') entries when labels was longer than choices. These phantom entries have no corresponding choice and are misleading.

This PR replaces zip_longest with explicit padding/truncation so that options() always returns exactly as many pairs as there are choices:

• Extra labels beyond the number of choices are silently ignored
• Missing labels are padded with empty strings (preserving existing behavior)

# Before
OneOf(choices=[], labels=["extra"]).options()  # [('', 'extra')]

# After
OneOf(choices=[], labels=["extra"]).options()  # []

All 1132 tests pass.

[lafrech]

LGTM. Thanks.

[lafrech]

Once this is merged, there will be only one call of zip_longest left, in _invoke_processors.

While removing it breaks a test, I find the use case questionable and I'd be tempted to suggest removing it in next major as allowing processors to modify the length of collections opens the door to unpredictable issues.