Fix nullable generation to use OpenAPI 3.1 type arrays instead of nullable: true — a GitHub Copilot (GitHub) bot flight

From the pull request description

Updates the nullable property generation from OpenAPI 3.0 style to OpenAPI 3.1 style, replacing `"nullable": true` with proper type arrays and `anyOf` constructs. ## Changes Made ### Before (OpenAPI 3.0 style): ```json { "type": "string", "nullable": true } ``` ### After (OpenAPI 3.1 style): …

Excerpt (first 300 characters) — the full description is on GitHub.