The getNextFallback function returned raw model names from the
hardcoded fallback chain without transforming them for the target
provider. For example, github-copilot requires dot notation
(claude-sonnet-4.6) but the fallback chain stores hyphen notation
(claude-sonnet-4-6).
The background-agent retry handler already calls
transformModelForProvider correctly, but the sync chat.message
hook in model-fallback was missing it — a copy-paste omission.
Add transformModelForProvider call in getNextFallback and a test
verifying github-copilot model name transformation.
