fix(cli): cdk flags --set without additional options should fail

[iankhou]

Fixes #832

Currently, running cdk flags --set or cdk flags --set --recommended is a no-op. Either the --all or --unconfigured options should be passed alongside --set. This can be attributed to insufficient testing during development of the feature.

Changes:

• Added validation to fix the problem above
• Brought flag-operations.ts to 100% unit test line coverage (previously 92%)

Validated with unit tests and manual testing.

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.67%. Comparing base (e7ea9d5) to head (55a7567).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #833      +/-   ##
==========================================
+ Coverage   83.05%   83.67%   +0.61%     
==========================================
  Files          65       65              
  Lines        9509     9514       +5     
  Branches     1122     1136      +14     
==========================================
+ Hits         7898     7961      +63     
+ Misses       1577     1529      -48     
+ Partials       34       24      -10     

Flag	Coverage Δ
suite.unit	83.67% <100.00%> (+0.61%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.