fix: generate simple {} schema for tools with no parameters

[DaleSeo]

fixes #303

Motivation and Context

This fix resolves the OpenAI compatibility issue reported in Issue #303.

Before

{
    "$schema": "http://json-schema.org/draft-07/schema#",
    "type": "object",
    "title": "EmptyObject",
    "description": "This is commonly used for representing empty objects in MCP messages..."
}

After

{}

How Has This Been Tested?

Updated existing test test_tool_macros_with_empty_param to validate the new behavior

Breaking Changes

None. This is a backward-compatible fix:

• Tools with parameters continue to generate proper complex schemas
• Only affects the schema generation for tools with no parameters

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[4t145]

Thanks for PR!