fix: resolve Pydantic serialization when switching to ollama model

The Ollama API returns datetime.datetime objects for the modified_at field,
but ModelInfo declares it as Optional[str], causing Pydantic validation
errors during model use commands.

Root cause: Type mismatch between Ollama API response data and ModelInfo type annotations
Solution: Convert datetime objects to strings at the model manager level

Fixes: llm:model:use commands failing with qwen3:0.6b and llama3.2:latest
Preserves: OpenAI API compatibility (cerebras/GLM-4.5-Air-REAP-82B-A12B-FP8 works)

Note: Additional code style changes applied by automated ruff formatter

Author: LittleCoinCoin