fix: preprocess JSON string params in Zod schemas for MCP compatibility

[Yanndotai]

Summary

• Some MCP clients (e.g. Claude Code) serialize array and object tool parameters as JSON strings instead of native JavaScript types
• This causes Zod validation to fail with Expected array, received string on n8n_update_partial_workflow, n8n_update_full_workflow, and n8n_create_workflow
• Added z.preprocess() wrappers that transparently parse JSON strings back to arrays/objects before validation, while leaving native types untouched

Affected schemas

• workflowDiffSchema → operations field
• createWorkflowSchema → nodes, connections fields
• updateWorkflowSchema → nodes, connections, settings fields

How it works

const jsonArrayPreprocess = (val: unknown): unknown => {
  if (typeof val === 'string') {
    try { const parsed = JSON.parse(val); if (Array.isArray(parsed)) return parsed; } catch {}
  }
  return val;
};

// Before: fails when MCP client sends string
operations: z.array(z.object({...}))

// After: transparently handles both string and native array
operations: z.preprocess(jsonArrayPreprocess, z.array(z.object({...})))

Test plan

• TypeScript build passes with no errors
• Tested with Claude Code MCP client: n8n_update_partial_workflow now accepts operations as JSON string
• Tested with Claude Code MCP client: n8n_update_full_workflow now accepts nodes and connections as JSON strings
• Native array/object parameters continue to work unchanged (preprocess is a no-op for non-string values)