## Summary
`unionCapabilities` merges `Experimental` map entries from inner variant
servers but skips the `Extensions` field entirely. This means any
extension registered via `ServerCapabilities.AddExtension()` on an inner
server (e.g. `io.modelcontextprotocol/skills`) is silently dropped from
the front proxy's `InitializeResult`.
- Add the same first-wins merge logic for `Extensions` that already
exists for `Experimental`
- Bump `go-sdk` to v1.4.1 which introduced the `Extensions` field on
`ServerCapabilities`
- Add unit test for `unionCapabilities` extensions merging
- Add integration test verifying extensions survive the variant path
end-to-end
## Test plan
- [x] `TestUnionCapabilities_MergesExtensions` — verifies first-wins
merge semantics for extensions
- [x] `TestIntegration_ExtensionsAdvertised` — verifies extensions
registered on inner servers appear in the client's `InitializeResult`
- [x] All existing tests pass (27/27)
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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