refactor: improve multimodal file handling architecture

- Make crewai_files an optional dependency with graceful fallbacks
- Move file formatting from executor to LLM layer (_process_message_files)
- Add files field to LLMMessage type for cleaner message passing
- Add cache_control to Anthropic content blocks for prompt caching
- Clean up formatters: static methods for OpenAI/Gemini, proper error handling
- Remove unused ContentFormatter protocol
- Move test fixtures to lib/crewai-files/tests/fixtures
- Add Azure and Bedrock multimodal integration tests
- Fix mypy errors in crew_agent_executor.py

lib/crewai/tests/cassettes/llms/TestAnthropicMultimodalIntegration.test_analyze_pdf.yaml

@@ -1,104 +0,0 @@
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lib/crewai/tests/cassettes/llms/TestGenericFileIntegration.test_generic_file_pdf_anthropic.yaml

@@ -1,104 +0,0 @@
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