🐛 Fix spurious 'Unknown option' warning for needs_string_links fields

[cpolzer]

Problem

Fields declared via needs_string_links options (e.g. 'options': ['github']) that are not also registered in needs_fields / needs_extra_options trigger a spurious needs.directive warning:

WARNING: Unknown option 'github' [needs.directive]

and the need is silently dropped from the build output.

Root cause

create_schema() in sphinx_needs/needs.py only populates the schema's extra field registry from needs_fields and needs_extra_options. The options names inside needs_string_links entries are never registered.

Both NeedDirective.run() and create_need() validate option keys against schema.iter_extra_field_names() — any key absent from the registry hits the rejection path.

Fix

After building the extra field schema from needs_fields, iterate needs_config.string_links and register any options field names not yet in the schema as nullable string extra fields. This is the correct layer — the schema is the single source of truth consulted by both the directive parser and create_need().

Fields already registered via needs_fields are skipped, so there is no conflict when a project declares the field in both places.

Tests

New file tests/test_string_links.py with three regression tests:

1. No warning — a field only in string_links options must not emit Unknown option
2. Value stored — the field value appears in needs.json (verified via the needs builder)
3. Guard — a truly unknown field (neither in needs_fields nor string_links) still warns

All existing test_needtable tests (which use needs_string_links with fields also in needs_fields) continue to pass.

Checklist

• Tests added
• tox -e ruff-check passes
• tox -e mypy passes
• Changelog updated (docs/changelog.rst, Unreleased > Bug fixes)