Go: implement system healthz API (#15307)

## Summary
- Add Go REST support for `GET /api/v1/system/healthz`.
- Return Python-compatible `ok`/`nok` dependency fields for DB, Redis,
document engine, and storage.
- Return HTTP 200 only when all checks pass; otherwise return HTTP 500
with `_meta` failure details.
- Add focused service coverage for the unhealthy dependency response
when Go dependencies are not initialized.

## Scope
This is a small, isolated slice of #15240. It avoids current open
connector PRs (#15274, #15300, #15265, #15264), tenant/member PRs
(#15295, #15301, #15276), MCP PRs (#15281, #15253, #15254, #15260,
#15261, #15262), and the memory-message PR (#15256).

Refs #15240