doc(macro): fix visibility attribute's usage of handler macro

[4t145]

Motivation and Context

fix the problem in #472

How Has This Been Tested?

Breaking Changes

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

[Copilot]

Pull Request Overview

This PR fixes documentation examples for the tool_router macro by correcting the syntax for the vis (visibility) attribute. The visibility attribute value should be wrapped in quotes as a string literal rather than being an unquoted identifier.

• Updates documentation examples to use proper string syntax for visibility attribute
• Adds a test case demonstrating the correct usage with quoted visibility values

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
crates/rmcp-macros/src/tool_router.rs	Adds test case with quoted visibility attribute
crates/rmcp-macros/src/lib.rs	Fixes documentation examples to use quoted visibility values
crates/rmcp-macros/README.md	Updates README examples to use proper quoted syntax

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