`appendSseHeaders()` was called inside Ktor's `sse {}` block where
response headers are already committed. On Netty this throws
`UnsupportedOperationException`, killing the SSE stream and causing the
client to retry in a loop
## How Has This Been Tested?
unit, simple-streamable-server sample
## Breaking Changes
none
## Types of changes
- [x] 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
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [x ] I have added appropriate error handling
- [ x] I have added or updated documentation as needed
8a7f9f8d31