fix: flag schema derive on schemars feature

[jamadeo]

Most derive(schemars::JsonSchema)s are flagged on the schemars feature, except for one instance (EmptyObject) which was flagged on server. This seems like a mistake, and means that enabling only schemars misses this one type.

Also changes the server feature to include the schemars feature itself, not just the dep. This does increate the server API, but preserves compatibility and I think aligns with the intention.

Motivation and Context

Makes rmcp buildable with only the schemars feature

How Has This Been Tested?

cargo check -p rmcp --no-default-features --features server
cargo check -p rmcp --no-default-features --features schemars

Breaking Changes

No 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

[DaleSeo]

Great catch, @jamadeo!