feat: add async resources for FastMCP

[micpst]

This PR adds support for async resource endpoints for fastmcp servers.

Motivation and Context

I noticed fastmcp doesn’t support asynchronous resources, even though the low-level API already has this functionality.

How Has This Been Tested?

• added unit test
• tested this on my private mcp server, which uses async resources

Breaking Changes

None

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

[dsp-ant]

This is great! Thank you!