fix(sdk): tidy go.mod and go.sum for direct oapi-codegen dep

The previous `feat(sdk): add codegen tools` change promoted oapi-codegen
from `// indirect` to a direct dependency. Running `go mod tidy` exposes
all of its transitive deps in go.sum. Without this, the CI `Tidy check`
gate (`go mod tidy && git diff --exit-code") reports the working tree as
dirty.

Author: factorydroid