Fixes a type change.

Motivation and Context

In #1692 I inadvertently changed the type of tool arguments from unknown to string

How Has This Been Tested?

Defined a sample CallToolRequest to ensure that any arguments are allowed:

const callToolRequest: CallToolRequest = {
  id: 'foo',
  jsonrpc: "2.0",
  method: "tools/call",
  params: {
    name: "foo",
    _meta: {
      any: 'no typescript error here',
      thing: {
        complex: 'object'
      }
    },
    arguments: {
      any: 1,
      thing: "here",
      at: {
        all: "not just strings"
      }
    },
    badKey: 'typescript error here - invalid key',
  }
}

Breaking Changes

No, it fixes an inadvertent breaking change.

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