Fix: Support nullable union types in Avro array items

[pcalcado]

Avro import failed when array items used nullable union types like ["null", "string"]. The importer could handle field-level unions but not array item unions.

Little bug that is blocking our research on adopting data contracts at Braze, I think I got a fix.

Example failing schema:

  {
    "type": "array",
    "items": ["null", "string"]  // ← This caused "Unsupported type union" error, should return "string" and make it not required
  }

Solution:

• Added union handling to import_avro_array_items() function
• Extracted shared union logic into import_type_from_union_schema()
• Reuses existing field-level union handling for consistency

• Tests pass
• ruff format
• README.md updated (if relevant)
•  CHANGELOG.md entry added

[jochenchrist]

LGTM