[nahapetyan-serob]

fixes #1480

[chrikrah]

@nahapetyan-serob nothing here blocks a merge from my side. With notifyListChanged stubbed to return nil, both new tests fail.

non-blocking: deleting server.go:1749 (ctx := context.WithValue(ctx, idContextKey{}, id)) keeps all of ./mcp green. Only Streamable HTTP routes by that ID (streamable.go:2093), and the tests use in-memory transports. Without it, a notify from inside a tool handler would go out on the tool call's stream, not the listen. A Streamable HTTP test that notifies from a handler would pin it.

$ go test ./... -count=1        # 4325744, go1.27.3 darwin/arm64, GOTOOLCHAIN=local
ok  	github.com/modelcontextprotocol/go-sdk/mcp	39.043s
(all 14 packages ok; go vet ./mcp clean)

$ (notifyListChanged returning nil first) go test ./mcp -run TestNotifyListChanged -count=1
--- FAIL: TestNotifyListChanged_OnlyChosenSession (15.01s)    # tool, prompt, resource
--- FAIL: TestNotifyListChanged_WhenSent (5.11s)               # legacy

$ (server.go:1749 replaced by `_ = id`) go test ./mcp -count=1
ok  	github.com/modelcontextprotocol/go-sdk/mcp	36.084s
# not run: a real HTTP client against a listen stream

Can that test go in with this PR?