Fix SSE server uncaught TypeError due to incorrect routing

[allenporter]

Motivation and Context

Update SSE server and examples to fix an uncaught TypeError due to incorrect routing:

TypeError: 'NoneType' object is not callable

These have been updated based on the discussions in #83

Fixes #101

How Has This Been Tested?

Tested with the following approaches:

• Approach manually tested in another project in Add library for serving an sse server proxying a stdio server sparfenyuk/mcp-proxy#8
• Tested uv run mcp-simple-tool --transport sse --port 8000 manually

These examples do not currently have automated test coverage, tracked in #109.

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

[dsp-ant]

Thank you!