feat(ciayn_agent): implement fallback mechanism in CiaynAgent to handle tool call failures and switch to alternative models
feat(__main__): add command line arguments for fallback configuration in the main application
feat(llm): add validation for required environment variables for LLM providers and merge chat histories during fallback
fix(config): define default values for maximum tool failures in configuration
test(ciayn_agent): add unit tests for fallback logic and tool call execution with retries and error handling
test(llm): enhance tests for LLM initialization and environment variable validation
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