feat(filesystem): support base64 write content

[kingrubic]

Summary

• add optional content_base64 to write_file so agents can submit JSON-safe base64 payloads
• decode UTF-8 content server-side while keeping existing content callers unchanged
• add an integration test that writes special characters through the MCP client using content_base64

Addresses #4394.

Tests

• npm run build --workspace @modelcontextprotocol/server-filesystem
• npm test --workspace @modelcontextprotocol/server-filesystem -- __tests__/structured-content.test.ts
• npm test --workspace @modelcontextprotocol/server-filesystem