Summary

• Reject move_file when the destination already exists instead of allowing POSIX fs.rename to replace it.
• Add real InMemory MCP regression coverage for both conflict preservation and successful moves to a new destination.
• Fixes [Bug Report]: filesystem move_file overwrites existing destination files despite its no-overwrite contract #1977.

The destination check uses lstat, so existing files, directories, and symlinks are protected. This is a preflight guard for the reported overwrite behavior; it does not claim an atomic cross-platform no-replace rename under a concurrent destination race.

Verification

• pnpm --filter @agent-infra/mcp-server-filesystem test (5 passed)
• pnpm exec prettier --check packages/agent-infra/mcp-servers/filesystem/src/server.ts packages/agent-infra/mcp-servers/filesystem/tests/server-in-memory.test.ts
• pnpm exec eslint packages/agent-infra/mcp-servers/filesystem/src/server.ts packages/agent-infra/mcp-servers/filesystem/tests/server-in-memory.test.ts
• pnpm exec tsc --noEmit -p packages/agent-infra/mcp-servers/filesystem/tsconfig.json
• pnpm --filter @agent-infra/logger build
• pnpm --filter mcp-http-server build
• pnpm --filter @agent-infra/mcp-server-filesystem build
• git diff --check

Checklist

• Added or updated necessary tests.
• Verified no breaking changes.