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Refactor generation options and normalize legacy parameters

- Updated the generation options schema in `lib/req_llm/generation.ex` to delegate to `ReqLLM.Provider.Options.generation_schema/0`, ensuring a unified approach for provider-specific options.
- Introduced normalization functions in `lib/req_llm/provider/options.ex` to handle legacy parameters, including `stop_sequences`, `thinking`, and `reasoning`, streamlining option processing across providers.
- Removed deprecated parameters from various provider implementations, enhancing code clarity and maintainability.
- Adjusted tests to reflect changes in option handling, ensuring compatibility with the new schema and normalization logic.