When ensureOpenClawModelsJson runs in merge mode, the shallow merge
{ ...existingProviders, ...providers } causes the main config to
overwrite agent-level models.json entries entirely. This drops any
apiKey or baseUrl that the user set specifically for an agent.
Preserve existing agent-level apiKey and baseUrl when merging, while
still accepting all other updates (model catalog, api type, etc.)
from the newly computed provider config.
Closes#27243
ee26a90bf5