fix(filesystem): reject existing move destinations

[dev-willbird1936]

Description

Make the filesystem server's move_file honor its documented no-overwrite contract. Existing destination files, directories, and symlinks now return a tool error, including when another process creates the destination during the move.

Server Details

• Server: filesystem
• Changes to: move_file

Motivation and Context

POSIX rename() can replace an existing destination. A separate existence check followed by rename() still has a check-to-use race, so a concurrent destination could be overwritten. The implementation now copies with exclusive destination creation and removes the source only after the copy succeeds.

How Has This Been Tested?

• Focused structured-content.test.ts suite: 13 passed.
• Full filesystem package suite: 155 passed.
• Filesystem package TypeScript build: passed.
• git diff --check: passed.
• LLM-client integration was not exercised; tests call the server tool through the MCP client test harness.

Breaking Changes

None to the documented API. A successful move now uses copy-then-remove rather than an atomic same-filesystem rename so that portable Node.js code can enforce no replacement. If copying fails, the source is retained.

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 follow MCP security best practices
• I have updated the server's README accordingly (not required; the documented contract is unchanged)
• 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 (not applicable)

Additional context

fs.cp() is called with force: false and errorOnExist: true, which makes destination creation exclusive and closes the overwrite race. Directory success and rejection without tree merging are covered by regression tests.