test(review): cover planner AI-Gateway route + title/body fallbacks

[JSONbored]

Summary

Test-only follow-up to #1360. That PR's head ref was stuck on the pre-fix commit when it merged (a GitHub PR-sync lag), so the two extra planner.ts coverage cases were left behind. This restores them:

• the AI_GATEWAY_ID route in runPlannerModel (the gateway extra is threaded as the 3rd AI.run arg);
• the title || "(none)" / body || "(no description provided)" fallbacks in generateIssuePlan.

With these, src/review/planner.ts is back to 100% line + branch coverage.

No linked issue — a coverage-completion follow-up to #1360.

Scope

• Test-only (test/unit/planner.test.ts) — no src/ change, so no codecov/patch obligation

Validation

• npm run typecheck — clean
• npx vitest run test/unit/planner.test.ts — 11 pass
• npm run test:coverage — planner.ts shows no uncovered lines and no partial branches

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.35%. Comparing base (4f52e3c) to head (83eb9e0).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1361   +/-   ##
=======================================
  Coverage   95.34%   95.35%           
=======================================
  Files         191      191           
  Lines       20652    20652           
  Branches     7460     7460           
=======================================
+ Hits        19690    19692    +2     
  Misses        378      378           
+ Partials      584      582    -2     

see 1 file with indirect coverage changes

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