feat(filesystem): add append_file

[altrao]

Description

Added append_file capability to filesystem server:

• Appends content to existing files or creates new files

Server Details

• Server: filesystem
• Changes to: tools

Motivation and Context

Need to provide log-style file operations for LLM clients:

• append_file enables incremental writes without full file rewrites

How Has This Been Tested?

Testing performed for append_file:

1. Basic functionality:
   • Append to existing files
   • Append to new files (creates file)
2. Security boundaries:
   • Works in allowed dir
   • Fails in not specified dir
3. Edge cases:
   • Empty files
   • Large content appends
   • Empty append to new file (creates empty file)
   • Empty append to existing file (maintains empty state)
4. Error handling:
   • Invalid paths
   • Permission issues

Breaking Changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Protocol Documentation
• My changes follows MCP security best practices
• I have updated the server's README accordingly
• I have tested this with an LLM client
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have documented all environment variables and configuration options

Additional context