fix(api): log the auto-combo variants GET /api/combos/auto skips instead of failing silently

[maxmad64bis]

⚠️ base-red inherited: #15306

Summary

Combo listing sometimes came back short with no log line saying which variant was missing. Each build loop dropped its own failure quietly, so a skipped variant left no trace. The endpoint now logs a single warning naming every variant that stayed out of the list and the first error, while the response stays the same.

Related Issues

• Related to fix(api): GET /api/combos/auto rebuilds the full provider pool 40x serially (~49s per request) #14889
• Related to fix(api): resolve auto-combo candidate capabilities once in GET /api/combos/auto #15353

Validation

• Change type: routing
• Focused tests and category gates from the golden path
• npm run lint — ESLint on the touched files is clean; the full run is red on the base (🔴 Release branch not green: release/v3.8.52 #15306).
• Reconciled with the current active release base; focused checks rerun afterward
• Production-code changes include a new or updated automated test in this PR

Tests Added Or Updated

• tests/unit/combos-auto-skipped-variants.test.ts — injected failure for a named, a template and a tiered-suffix variant (one line, id named once, variant absent), a variant that fails once and builds in a later loop (kept, not reported), the quiet case, payload stability, and a bounded single-line message.
• Failure injection poisons one scoring weight pack, so the tests run in CI without module mocks; it reaches the named, template and tiered-suffix loops. The model-family loop has no such seam and goes through the same helper and the same catch.

Coverage Notes

• src/app/api/combos/auto/route.ts — covered by tests/unit/combos-auto-skipped-variants.test.ts (injected failures, retry across loops, quiet case, payload stability, message bounding).

Reviewer Notes

• Behavior: Only the log is new: an id is still marked as seen once its build succeeds, so a variant that fails in one loop can be built by a later loop (six ids are built by two loops). The warning lists each missing id once, keeps the first error on one line and cuts it at 300 characters.
• Merge: Either order with fix(api): resolve auto-combo candidate capabilities once in GET /api/combos/auto #15353, the second to land rebases on the release tip. This change keeps the preparation block untouched and only edits the loops and the final warning, so a merge stays mechanical.
• Follow-up: Per-request capability resolution and memoization stay open for a later change; this one only logs skipped variants.