feat(mux): mint agent gateway tokens for running devboxes (#10872)

Empty by design. The generated code for POST /v1/devboxes/{id}/create_gateway_token
is already on this branch — it arrived via 9591d74 ("initial stlc build"), which
regenerated the whole SDK from a spec that already contained the endpoint.

This commit exists only to restore changelog attribution. The cloud pipeline
recorded this feature as b7763ba on the now-abandoned `next` branch, and
release-please reads commit messages rather than diffs, so without it the 1.31.0
changelog credits the API addition to a "Build SDK" commit instead.

Author: tode-rl