feat: add tool_choice parameter to chat() (closes #663)

[shivanireddyk]

Closes #663

Adds tool_choice support to chat() (sync and async), mirroring the OpenAI API and the server's existing partial support.

Changes

• New ToolChoiceFunction class and ToolChoice type alias in _types.py
• Both exported from top-level ollama package
• tool_choice field added to ChatRequest
• All 6 chat() overloads updated (3 sync + 3 async) with typed parameter

Usage

from ollama import ToolChoiceFunction

# Forcing a specific tool
ollama.chat(
  model='llama3.2',
  tools=[get_weather],
  tool_choice=ToolChoiceFunction(
    function=ToolChoiceFunction.Function(name='get_weather')
  ),
  messages=[{'role': 'user', 'content': 'What is the weather in Paris?'}],
)

# It requires the model to call *some* tool
ollama.chat(model='llama3.2', tools=[...], tool_choice='required', messages=[...])