fix: declare zod runtime dependencies

[he-yufeng]

Fixes #4288.

Summary

@modelcontextprotocol/server-sequential-thinking imports zod directly from its runtime entrypoint, but the package did not declare zod as a dependency. With strict package isolation, consumers can install the server without a local zod, then hit ERR_MODULE_NOT_FOUND at runtime.

While checking the same dependency boundary, I found two other published server packages with the same direct runtime import:

• @modelcontextprotocol/server-memory
• @modelcontextprotocol/server-filesystem

This patch declares zod in those three packages, using the same ^4.0.0 range already used by @modelcontextprotocol/server-everything.

Validation

• npm install --package-lock-only --ignore-scripts
• npm install --ignore-scripts
• npm run build --workspace=@modelcontextprotocol/server-sequential-thinking
• npm run build --workspace=@modelcontextprotocol/server-memory
• npm run build --workspace=@modelcontextprotocol/server-filesystem
• npm test --workspace=@modelcontextprotocol/server-sequential-thinking
• npm test --workspace=@modelcontextprotocol/server-memory
• npm test --workspace=@modelcontextprotocol/server-filesystem
• npm exec prettier -- --check src/filesystem/package.json src/memory/package.json src/sequentialthinking/package.json package-lock.json
• git diff --check