fix(runtimes): hide private runtimes from non-owners (#7435)

* fix(runtimes): enforce private runtime access

* fix(runtimes): gate agent model discovery

* refactor(runtimes): reuse isRuntimeUsableForUser on runtime detail

The detail page re-derived the owner-or-public rule inline instead of
calling the shared helper every other surface in this PR uses. The two
disagreed on an ownerless runtime: the helper is fail-closed (matching
the server's read gate, which reuses canUseRuntimeForAgent), while the
inline copy rendered Usage for a public one and could only 404.

Co-authored-by: multica-agent <github@multica.ai>

---------

Co-authored-by: m2keloop <lanqing@echo.tech>
Co-authored-by: Bohan-J <bohan.optimism@gmail.com>
Co-authored-by: multica-agent <github@multica.ai>