chore: run the mock server from locked local tooling (#3679)

# Summary

This is part of a series to reduce the SDK's dependency surface and make
contributor tooling more predictable. It builds on the merged #3678.

- Lock the existing Steady 0.22.1 release and its platform packages in
the repository's pnpm lockfile.
- Keep `./scripts/bootstrap` followed by `./scripts/test` working,
including automatic mock-server startup, an existing server,
`TEST_API_BASE_URL`, and both Pydantic test lanes.
- Run only the locally installed mock server. Missing tooling produces
setup instructions instead of an on-demand npm download.

This completes the mock-tooling portion of #3116 using the current
uv/pnpm setup.