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Refactor API key management and enhance provider implementations

- Replaced the `ReqLLM.Key` module with `ReqLLM.Keys` for improved API key retrieval logic, consolidating key fetching with a clear precedence order.
- Updated provider modules (`Anthropic`, `Google`, `Groq`, `OpenAI`, `OpenRouter`, `XAI`) to utilize the new `ReqLLM.Keys.get!/2` function for API key management.
- Enhanced documentation in `README.md` and `guides/getting-started.md` to clarify API key configuration options and their precedence.
- Removed the deprecated `ReqLLM.Key` module and added comprehensive tests for the new `ReqLLM.Keys` module to ensure correct functionality and error handling.