fix: preserve nested workSkills/workSkillGroups fields on CapacityCategory

[be-ant]

Summary

• The CapacityCategory model silently dropped fields from its nested workSkills / workSkillGroups items (they were typed as the thin Item{label, name}). ratio, startDate, end_date, audit fields, and (for groups) links were lost. Discovered by capturing live responses from a real OFS instance.
• Added dedicated CapacityCategoryWorkSkill and CapacityCategoryWorkSkillGroup models with the documented fields and extra="allow" so audit/future fields are preserved.
• Flipped CapacityCategory from extra="ignore" to extra="allow" to stop silent loss at the container level too (aligns with Audit and tighten Pydantic models using extra="allow" to prevent silent data loss #149).
• timeSlots left unchanged (API returns label-only items).
• Bumped version to 2.26.2.

Fixes #175

Test Plan

• New unit tests in tests/metadata/test_capacity_categories.py assert ratio/startDate/end_date are typed, audit fields survive via model_extra and round-trip through model_dump(), workSkillGroups[].links parse, and empty nested lists parse cleanly. These use inline data (no credentials), so they run in CI.
• Updated tests/test_model.py::test_capacity_category_model_list to assert the now-preserved nested fields (the previously commented-out assertion).
• Full suite: 445 passed, 8 skipped (credential/local-data tests); ruff clean.
• Validated against a live captured response: all 43 categories parse with nested fields populated.

Compatibility note

workSkills / workSkillGroups change from ItemList (a RootModel) to plain list[...]. Iteration and indexing are unaffected; only isinstance(x, ItemList) checks on these two attributes would change (none exist in the tree).