This PR introduces a new runtime type‐guard, isZodType, which uses duck‐typing (inspecting the internal _def.typeName) instead of instanceof or any to reliably detect ZodTypeAny objects—even when multiple copies of Zod are loaded.

Motivation and Context

When bundling or installing Zod in more than one place (e.g. your app plus @vercel/mcp-adapter), using instanceof ZodType can fail because each bundle has its own constructor/prototype.
This change:

• Eliminates the brittle instanceof check
• Avoids the use of any by starting from unknown and narrowing step by step
• Provides clear documentation for future maintainers

How Has This Been Tested?

• Unit tests covering positive and negative cases for isZodType (various Zod schema instances and non-Zod objects)
• Manual smoke tests in a sample app with multiple Zod versions loaded

Breaking Changes

None. Existing code using isZodRawShape or instanceof ZodType will continue to work, and consumers who switch to the new guard will see no change in external behavior.

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

Fix of #467 and vercel-labs/mcp-for-next.js#9