When the server returns a model string that already contains a provider
prefix (e.g. "ollama/gpt-oss:120b-cloud"), buildQualifiedChatModelValue
would blindly prepend the session's default provider, producing invalid
refs like "anthropic/ollama/gpt-oss:120b-cloud".
Skip provider prepending when the model string already contains "/".
Closes#51139Closes#51306
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>