Fix n8n compatibility in Plumber API schema

[armish]

This commit addresses JSON schema format incompatibilities with n8n's built-in MCP node. The changes ensure proper data streaming and validation when using plumber2mcp with n8n workflow automation.

Changes:

1. Convert 'required' field from character() to list() in inputSchema

   • n8n expects 'required' to be a JSON array, not a character vector
   • Empty required fields now serialize as [] instead of []

2. Comment out outputSchema from tool definitions

   • n8n's structured data validation fails when outputSchema is present
   • The schema was causing validation errors for text responses
   • Tools still function correctly without outputSchema defined

3. Update all test expectations to match new schema format

   • test-enhanced_schemas.R: Update required field type checks
   • test-security_validation.R: Remove outputSchema expectations
   • test-edge_cases.R: Already compatible with list-based required
   • test-pr_mcp.R: Already compatible with list-based required

Impact:

• Existing functionality preserved
• Better n8n integration out of the box
• No breaking changes to API endpoints or MCP protocol
• Tests updated to reflect new schema format

Fixes compatibility with n8n MCP node HTTP streaming transport.