Step c in switch_model() blindly converted the first colon to a slash for
aggregator providers, even when the model name already contained a slash
(vendor/model format). This mangled variant tags like :free into /free,
causing 400 Bad Request from the API.
Fix: skip the colon→slash conversion when the model already has a slash,
since the colon is a variant tag, not a vendor separator. The module
docstring already documented this intent (line 17-18) but the
implementation didn't enforce it.
Reported via Discord. Related to PR #6088 (which identified the same bug
but placed the fix in model_normalize.py instead of model_switch.py where
the actual mangling occurs).
980fadfea9