Fix isZodRawShape implementation to properly handle the empty object and add the test.

Close #453

Motivation and Context

The isZodRawShape type guard function was incorrectly returning false for the empty object. This caused issues when trying to register tools with empty parameters. The fix ensures that empty objects are correctly identified as valid ZodRawShape instances, which allows for proper tool registration with empty parameter schemas.

How Has This Been Tested?

Added a new test case in mcp.test.ts that specifically tests registering a tool with an empty parameters object. The test verifies that the tool is correctly registered with an empty object schema and that the proper annotations are applied.

Additionally, manual testing regarding types accept by ZodRawShape was done with a simple test snippet:

import { type ZodRawShape } from 'zod';

class Test {
  zodRawShape(input: ZodRawShape) {
    console.log(input);
  }
}

const test = new Test();
test.zodRawShape({}); // type check succeed - now works correctly on our type guard
test.zodRawShape(null); // type check failed as expected

Breaking Changes

None. This is a bug fix that maintains backward compatibility.

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

The fix modifies the isZodRawShape type guard to explicitly handle empty objects as valid ZodRawShape instances. This is done by using Zod's own validation to check if the object is an empty object. If it is, we consider it a valid ZodRawShape. Otherwise, we fall back to the original check which verifies if any property is a ZodType instance.

The issue was particularly important for tools that don't require any parameters but still need to be properly registered with the MCP server.