Add middleware to support Claude's native search + text editor tools

[rlancemartin]

Implements Claude's native text editor tool as an alternative to the standard FilesystemMiddleware.

This provides exact compatibility with Claude's text_editor_20250728 -- https://docs.claude.com/en/docs/agents-and-tools/tool-use/text-editor-tool

Key features:

• Single command-based tool: str_replace_based_edit_tool
• Four commands: view, str_replace, create, insert
• Reuses existing backend system (StateBackend, FilesystemBackend, StoreBackend)
• Configurable via use_claude_native_text_editor parameter
• Full test coverage with 19 passing tests