Add comprehensive test suite for Ollama chat response parsing and provider implementation

- Add test module for OllamaChatResponse deserialization: cover basic responses, usage info, tool calls, thinking mode, done_reason variants, and optional fields
- Add test module for parse_response() function: test text content parsing, stop reason handling (stop/length/unknown), usage info extraction, tool call parsing (single/multiple), thinking content handling, and response metadata
- Add test module for Chat

Author: gitctrlx