move RouterService to examples

[itsaphel]

Motivation and Context

Move an example from the core crate to the examples directory. Follows up on #6 and #10

Also a minor syntactical update to crates/mcp-server/src/lib.rs (can make the match expr more idiomatic)

How Has This Been Tested?

It compiles and runs:

• cargo run -p mcp-server-examples --example counter-server
• cargo run -p mcp-server-examples --example axum

Breaking Changes

No

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

[itsaphel]

never mind - it seems right where it is. too early in the morning 😅