fix(distributed): pass ExternalURI through NATS backend install (#9446)

When installing a backend with a custom OCI URI in distributed mode,
the URI was captured in ManagementOp.ExternalURI by the HTTP handler
but never forwarded to workers. BackendInstallRequest had no URI field,
so workers fell through to the gallery lookup and failed with
"no backend found with name <custom-name>".

Add URI/Name/Alias fields to BackendInstallRequest and thread them from
ManagementOp through DistributedBackendManager.InstallBackend() and the
RemoteUnloaderAdapter. On the worker side, route to InstallExternalBackend
when URI is set instead of InstallBackendFromGallery. Update all
remaining InstallBackend call sites (UpgradeBackend, reconciler
pending-op drain, router auto-install) to pass empty strings for the
new params.

Assisted-by: Claude Code:claude-sonnet-4-6

Signed-off-by: Russell Sim <rsl@simopolis.xyz>