fix: use fallback chain when connected-providers cache missing

[evsong]

Summary

• Fix category model configurations being ignored when connected-providers.json cache doesn't exist
• Use first fallback chain entry instead of skipping to systemDefaultModel

Problem

When connected-providers.json cache is missing (first run or cache cleared), resolveModelWithFallback() was skipping the entire fallback chain and falling through to systemDefaultModel. This caused all category-specific model configurations (quick, visual-engineering, ultrabrain, etc.) to be ignored.

Related Issues: #1264, #1295

Solution

When cache is null, use the first entry in the fallback chain instead of skipping:

if (connectedSet === null) {
  const firstEntry = fallbackChain[0]
  if (firstEntry && firstEntry.providers.length > 0) {
    const model = `${firstEntry.providers[0]}/${firstEntry.model}`
    return { model, source: "provider-fallback", variant: firstEntry.variant }
  }
}

Testing

• Updated 2 existing tests to reflect new behavior
• All 46 tests pass

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Summary by cubic

Fixes category-specific model selection when connected-providers.json is missing by using the first fallback chain entry instead of the system default. Ensures categories like quick, visual-engineering, and ultrabrain use their intended models on first run or after cache clear.

• Bug Fixes
  • When no connected-providers cache exists, resolveModelWithFallback now picks the first fallback entry (preserving variant) and marks source as provider-fallback.
  • Updated tests to reflect the new behavior.

^{Written for commit 7ef706f. Summary will update on new commits.}

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[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.