fix(schema): remove AddNullable from draft2020_12 settings

[mwotton]

Problem

The schema_for_type() function uses AddNullable::default() as a transform when generating JSON Schema 2020-12 schemas. However, the nullable keyword is an OpenAPI 3.0 extension, not part of JSON Schema 2020-12. This causes validation failures with strict JSON Schema validators.

Solution

Remove the AddNullable transform from the draft2020_12 settings. JSON Schema 2020-12 represents nullable types using:

• {"type": ["string", "null"]} (type array with null)
• {"anyOf": [{"type": "string"}, {"type": "null"}]}

Before

An optional string field produced:

{
  "nullable": true,
  "type": "string"
}

After

An optional string field produces:

{
  "type": ["string", "null"]
}

Fixes #663

[alexhancock]

@mwotton Thanks for the fix. test_complex_schema needs a look

[mwotton]

Added a second follow-up commit (70011f9) after the test_complex_schema fix because another test had the same stale nullable assumption.

test_tool_macros::test_optional_field_schema_generation_via_macro was still asserting OpenAPI-style nullable: true and explicitly rejecting type: ["string", "null"], which conflicts with Draft 2020-12 output from schema_for_type().

This was not visible in the original CI failure because cargo test --all-features failed fast on test_complex_schema first. The second commit aligns that macro test with Draft 2020-12 behavior.