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Add fake API key support for testing

- Introduced `ReqLLM.TestSupport.FakeKeys` module to inject fake API keys when not in LIVE mode, ensuring unit tests can run without real API keys.
- Updated `test/test_helper.exs` to call the new `install!` function, enhancing the testing workflow by providing mock keys for all registered providers.
- Added `:api_key` to the internal keys in `ReqLLM.Provider.Options` to bypass validation during testing.

These changes improve the testing environment, allowing for more flexible and isolated unit tests without affecting production code.