fix: gate client handler bounds for local

[DaleSeo]

Fixes #1062.

Motivation and Context

This gives ClientHandler feature-gated trait bounds, allowing handlers with single-threaded state when the local feature is enabled while retaining the existing Send + Sync signature otherwise. It also adds a compile-time regression test using Rc.

How Has This Been Tested?

Added a regression test

Breaking Changes

None. Builds without local retain the existing Send + Sync requirements.

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

[gold-silver-copper]

awesome thx for the quick fix