bug: Fix Pydantic compatibility for OpenAI chat message validator

Summary

nemoguardrails/server/schemas/openai.py uses BeforeValidator(..., json_schema_input_type=...) for OpenAIChatMessage.

The project declares pydantic>=2.5,<3.0. Pydantic 2.5.0 does not support the json_schema_input_type argument on BeforeValidator. Importing the server schema can therefore raise TypeError for a supported dependency version.

Required change

Keep the OpenAI chat-message validator compatible with the declared Pydantic range. Either remove or replace the unsupported json_schema_input_type argument, or raise the minimum supported Pydantic version and regenerate uv.lock with the project tooling.

Affected area

• nemoguardrails/server/schemas/openai.py
• Pydantic dependency declaration and lockfile, if the minimum version changes

Acceptance criteria

• The server schema imports successfully with every supported Pydantic version.
• The OpenAI chat-message validation still requires a string role.
• JSON Schema generation remains correct for the request model.
• Tests cover the selected compatibility approach.

Backlinks

• Pull request: https://github.com/NVIDIA-NeMo/Guardrails/pull/2266
• Review comment: https://github.com/NVIDIA-NeMo/Guardrails/pull/2266#discussion_r3795104541
• Requested by: @Pouyanpi