The no-auth OpenCode provider has id "opencode" and alias "oc". The combo
builder built qualifiedModel from the provider id ("opencode/big-pickle"), but
parseModel("opencode/...") resolves to the opencode-zen api-key tier via a
manual ALIAS_TO_PROVIDER_ID override — not the no-auth "opencode" provider.
"oc/<model>" resolves correctly.
Rewrite qualifiedModel to the provider alias for no-auth providers (only when
the alias differs from the id), keeping providerId for getModelIsHidden. Aligned
the route test that previously asserted the buggy "opencode/" prefix.
Closes#2901
