refactor: improve AgentConfig file handling and default tools

- Add static FILE_PREFIX constant for better file:// handling
- Use removeprefix() instead of substring indexing for cleaner code
- Add static DEFAULT_TOOLS list with experimental minimum viable tools
- Add comprehensive error handling for missing strands_tools
- Require either strands_tools installation or custom ToolPool with your own tools
- Fix logic to check for tools before creating default ToolPool
- Update all tests to handle new validation requirements
- Add test for ImportError when strands_tools not available

🤖 Assisted by Amazon Q Developer

Author: mrlee-amazon

src/strands/experimental/agent_config.py

@@ -82,7 +82,7 @@ def _create_default_tool_pool(self) -> ToolPool:
                 raise ImportError(
                     f"strands_tools is not available and no ToolPool was specified. "
                     f"Either install strands_tools with 'pip install strands-agents-tools' "
-                    f"or provide your own ToolPool with native tools."
+                    f"or provide your own ToolPool with your own tools."
                 )
 
         return pool