fix(prompt): update OpenAI JSON schema generation for serialization of nullable collections

[grantlerduck]

Summary

• Nullable List<X>? fields produced {"type": ["array", "null"], "items": {...}}, which OpenAI's
  GPT-5-family strict-mode validator rejects (surfaces as 'Invalid schema for response_format ... got "type": "None"'). Switch OpenAIStandardJsonSchemaGenerator.processList to emit
  {"anyOf": [{"type": "array", "items": ...}, {"type": "null"}]} for nullable lists, mirroring
  the shape already used for nullable objects.
• Non-nullable lists, nullable primitives, and the LLM-agnostic StandardJsonSchemaGenerator are
  unchanged. Only the OpenAI-specific generator is touched, so other providers keep their existing
  type-union encoding.
• Adds a regression test (testGenerateOpenAIStandardJsonSchemaForNullableLists) covering both
  primitive (List<String>?) and $ref (List<Tag>?) item types.

Out of scope (could follow up)

• OpenAIBasicJsonSchemaGenerator still emits the OpenAPI-3.0-style nullable: true keyword, which
  OpenAI strict mode silently ignores. Same root cause family, different generator — happy to file
  a follow-up if useful.

closes #1930

[grantlerduck]

Just rebased. There was an agent run test which ran into a deadlock unrelated to my changes.