`ServerSession.handleInitialize()` now rejects duplicate `initialize`
requests with `INVALID_REQUEST` (-32600) error. `clientCapabilities` and
`clientVersion` changed from `var` to `AtomicRef`-backed `val`, with
atomic `compareAndSet(null, ...)` serving as both the guard and the
write
## How Has This Been Tested?
second `initialize` returns error, original capabilities preserved
## 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
## Additional context
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