test(signals): fold reward-risk-engine-branch-coverage.test.ts into its siblings

[JSONbored]

Closes #8578.

Summary

reward-risk-engine-branch-coverage.test.ts was a #2281 Codecov bolt-on testing buildRepoRewardRisk / rewardRiskFreshnessInternals / buildContributorRewardRiskStrategy from its own disconnected file. Moved each case to the sibling that already owns the function under test:

• bestFitLabels tie-break -> reward-risk-freshness.test.ts's existing bestFitLabels keyword anchoring describe (reused its pick() idiom)
• reviewChurnRisk (high/medium) + maintainer-cut readiness -> reward-risk-freshness.test.ts, new describe block
• buildContributorRewardRiskStrategy tie-break -> reward-risk-reports.test.ts, new describe block (the only reward-risk sibling besides the signals-v2/signals-coverage family, which stays out of scope here per Epic: consolidate the '-coverage' bolt-on test-file family #8574/Consolidate signals-coverage.test.ts into the signals main suites #8576)

Test plan

• npx vitest run test/unit/reward-risk-freshness.test.ts test/unit/reward-risk-reports.test.ts test/unit/reward-risk-competition-fail-closed.test.ts — 34/34 pass
• npm run typecheck — clean
• Coverage regression check, before (siblings + bolt-on) vs after (siblings merged): identical on both touched modules —
  • packages/loopover-engine/src/reward-risk.ts: 259/303 branches (85.47%), 152/164 lines (92.68%)
  • packages/loopover-engine/src/signals/engine.ts: 441/1973 branches (22.35%), 437/1351 lines (32.34%)

[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 91.99%. Comparing base (346c183) to head (2ffb737).
⚠️ Report is 6 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8582      +/-   ##
==========================================
- Coverage   92.54%   91.99%   -0.56%     
==========================================
  Files         796      796              
  Lines       79845    79845              
  Branches    24134    24134              
==========================================
- Hits        73896    73452     -444     
- Misses       4802     5309     +507     
+ Partials     1147     1084      -63     

Flag	Coverage Δ
backend	92.95% <ø> (-0.78%)	⬇️

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see 3 files with indirect coverage changes