feat(filesystem): add append_file and write_or_update_file tools

[agn-7]

Description

Fixes #2807

Added two new tools to the filesystem MCP server for enhanced file modification capabilities:

• append_file: Appends content to existing files without overwriting
• write_or_update_file: Creates new files or appends to existing ones (smart create-or-append)

These tools complement the existing write_file tool by providing non-destructive file operations, addressing the common use case where users want to add content to files while preserving existing data.

Server Details

• Server: filesystem
• Changes to: tools (added 2 new tools), core library functions, tests

Motivation and Context

The existing write_file tool always overwrites file content, which is problematic when users want to:

• Add content to existing log files or documents
• Incrementally build up file content
• Preserve existing data while adding new information

These new tools solve this by providing:

1. append_file: Safe appending to existing files (fails if file doesn't exist)
2. write_or_update_file: Flexible create-or-append operation (creates if needed, appends if exists)

How Has This Been Tested?

• ✅ Tested with MCP client using local build
• ✅ All unit tests pass (51/51)
• ✅ Tested scenarios:
  • Appending to existing files
  • Creating new files with write_or_update_file
  • Appending to existing files with write_or_update_file
  • Error handling (file not found, permission errors)
  • Atomic write operations with concurrent access

Breaking Changes

No breaking changes. These are new tools that don't affect existing functionality.

Types of changes

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

Checklist

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

Additional context

Implementation details:

• Both functions use atomic write operations (temp file + rename) for safety
• Follow existing patterns from the codebase for consistency
• Proper error handling and cleanup of temporary files
• Comprehensive test coverage added (6 new test cases)
• Tool descriptions clearly explain use cases and differences

[agn-7]

Hi

I really need this feature. could you please take a look?

@olaservo @sebastien-rosset @Finndersen @domdomegg