Add optional description field to Implementation struct

[DaleSeo]

Motivation and Context

The MCP spec 2025-11-25 added an optional description field to the Implementation interface in modelcontextprotocol/modelcontextprotocol/pull/1817. This provides a human-readable description of what an implementation does, giving clients and servers richer context during the initialization handshake.

The Rust SDK already supports the other Implementation metadata fields (title, icons, websiteUrl) but was missing description.

How Has This Been Tested?

Updated the exiting tests to account for the changes.

Breaking Changes

Deserialization is not affected; existing JSON without description will still parse correctly.
However, code that constructs Implementation with struct literal syntax without ..Default::default() will need to add description: None (or a value). This is the same kind of breakage that occurred when title, icons, and website_url were added.

// This will need updating:
Implementation {
    name: "my-server".to_string(),
    version: "1.0.0".to_string(),
    title: None,
    icons: None,
    website_url: None,
    // description: None,  // <-- add this
}

// This continues to work as-is:
Implementation {
    name: "my-server".to_string(),
    version: "1.0.0".to_string(),
    ..Default::default()
}

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