fix: switch to Ollama OpenAI-compatible endpoint to resolve tool call validation errors

[SusieGlitter]

1. Related Issue
   Closes Ollama integration fails with tool calls: "role" field missing in Message validation #2

2. Reproduction Process & Results
   I access ollama-0.21.0 on linux server with SSH Port Forwarding while running opencook windows PC, so I can use ollama as model provider with localhost:11434

using opencook_config.yaml with config:

...
...

model_providers:
    ollama:
        base_url: http://localhost:11434/v1
        api_key: ollama
        provider: ollama

models:
    plan_agent_model:
        model_provider: ollama
        model: qwen3.5:cloud

...
...

running opencook in powershell:

python -m code_agent interactive --config-file opencook_config.yaml --working-dir "G:\db\opencook\sqlite-src-3530000"

and result is after entering the command, ollama serve show that no request is sent and no step is shown in opencook cli

3. Changes
   Refactored ollama_client.py: Switched from the native Ollama library/endpoint to the OpenAI-compatible /v1/chat/completions endpoint.

Improved Message Formatting: By using the OpenAI protocol, messages (including tool roles and tool_outputs) are now automatically formatted correctly according to standard schemas.

4. Final Performance & Verification
   doing the same in 2. Reproduction Process & Results, and opencook runs as expected