test: cover flag utilities

[w3lld1]

PR Checklist

• Addresses an existing open issue: Fixes 🧪 Tests: Add tests for remaining functions in src/flags.ts #13
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Adds focused true/false coverage for every exported flag utility in src/flags.ts, including modifier, node, object, symbol, transient-symbol-link, and type flags.

Validation:

• pnpm exec vitest run src/flags.test.ts (6 passed)
• pnpm test --run (321 passed)
• pnpm lint
• pnpm tsc --noEmit
• pnpm build
• git diff --check

💖

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.82%. Comparing base (e59a11b) to head (bf3aa9b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1050   +/-   ##
=======================================
  Coverage   58.82%   58.82%           
=======================================
  Files          29       29           
  Lines         901      901           
  Branches      283      283           
=======================================
  Hits          530      530           
+ Misses        287      286    -1     
- Partials       84       85    +1     

Flag	Coverage Δ
5.5.4	58.82% <ø> (+0.11%)	⬆️
5.9.3	58.82% <ø> (+0.11%)	⬆️
latest	?

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[JoshuaKGoldberg]

https://typescript-eslint.io/contributing/ai-policy/