Add OpenAI-compatible provider for agentic tools

[SufianTA]

Summary

• add OPENAI as a first-class AgenticTool provider backed by the OpenAI SDK
• support OpenAI-compatible endpoints via OPENAI_BASE_URL, plus model-token override env vars
• document provider selection and add unit coverage for client initialization/inference config

Closes #8.

Tests

• python -m pytest -q --no-cov tests\unit\test_openai_compatible_client.py tests\unit\test_agentic_tool_env_vars.py
• python -m compileall -q src\tooluniverse tests\unit\test_openai_compatible_client.py
• python -m json.tool src\tooluniverse\data\agentic_tools.json > $null
• git diff --check

Notes

• Ruff is configured in the project, but python -m ruff is not available in this local environment.