🧹 Refactor duplicated Import Error boilerplate in LLM Providers

[leo-gan]

This PR improves the maintainability and readability of the LLM provider implementations by standardizing how optional dependency imports are handled.

🎯 What:

• Introduced a validate_import(extra_name) decorator in pdf_anonymizer_core.llm_provider.
• Replaced repetitive try...except ImportError blocks in GoogleProvider, OllamaProvider, HuggingFaceProvider, OpenRouterProvider, OpenAIProvider, and AnthropicProvider with the new decorator.
• Added comprehensive tests for these error conditions.

💡 Why:
The previous implementation had a lot of boilerplate code for each provider to check if its required package was installed. This refactoring centralizes that logic, making it easier to add new providers and ensuring a consistent user experience when an optional extra is missing.

✅ Verification:

• New tests in tests/test_llm_provider_imports.py verify that the correct ImportError with the expected message is raised for each provider when its module is unavailable. These tests mock sys.modules to be environment-independent.
• Existing tests in tests/test_main.py (using mocks for the LLM calls) continue to pass.

✨ Result:
Cleaner, more maintainable code with a standardized pattern for handling optional dependencies across all LLM providers.

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PR created automatically by Jules for task 11010072440898765876 started by @leo-gan

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[gemini-code-assist]

Code Review

This pull request introduces a validate_import decorator in llm_provider.py to standardize the handling of ImportError for optional LLM provider dependencies. Several provider classes, including GoogleProvider, OllamaProvider, and OpenAIProvider, have been refactored to use this decorator, which simplifies their initialization logic by removing repetitive try-except blocks. Additionally, a new test suite tests/test_llm_provider_imports.py has been added to verify that the correct error messages are raised when these optional dependencies are missing. I have no feedback to provide.