refactor: extract shared ADO logging helper for latestVersions commands

[tylerbutler]

Summary

• Extracts the duplicated ADO variable-setting and logging block from check:latestVersions and vnext:check:latestVersions into a shared logLatestVersionResult helper in src/library/latestVersions.ts
• Both commands now call the helper instead of inlining the same ~30 lines of ##vso[task.setvariable ...] output logic
• Adds unit tests for the new helper covering all three branches (latest, not latest, no matching major)
• The deprecated check:latestVersions command remains deprecated and unchanged in behavior

Fixes #26560
Fixes #26943

Test plan

• Unit tests for logLatestVersionResult added and passing (3 new tests)
• Existing isLatestInMajor unit tests still pass (7 tests)
• Integration tests for vnext:check:latestVersions still pass (3 tests)
• Full test suite passes (488 passing)
• Biome formatting check passes
• TypeScript compilation succeeds with no new errors