fix: merge Extensions in unionCapabilities (#9)

## 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>

.github/workflows/go.yml

@@ -24,7 +24,7 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@7a3fe6cf4cb3a834922a1244abfce67bcef6a0c5 # v6.2.0
         with:
-          go-version: "^1.25"
+          go-version: "^1.24"
       - name: Check formatting
         run: |
           cd go/sdk
@@ -51,7 +51,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go: ["1.23", "1.24", "1.25"]
+        go: ["1.24", "1.25"]
     steps:
       - name: Check out code
         uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1