Reorganize Test Structure and Update Project Layout

This PR reorganizes the test structure by moving all provider tests from the core module to a dedicated tests/core-providers directory. This change improves the project organization and separates test code from the core implementation.

Key changes:

• Created a new tests/core-providers directory with comprehensive test suite for all supported providers
• Added detailed documentation in tests/core-providers/README.md explaining how to run tests
• Removed test dependencies from core/go.mod (godotenv, testify, etc.)
• Updated README.md to include the new tests directory in the project structure
• Expanded model lists in the example configuration
• Fixed OpenAI router path from /openai/v1/chat/completions to /openai/chat/completions

The new test structure provides better organization and makes it easier to run tests for specific providers or scenarios. The comprehensive test suite covers all major functionality including chat completion, tool calling, image processing, and multi-turn conversations.