[agent] feat: add vertical tab symbol utility

[FurinaDog]

Summary

• add the dependency-free isVerticalTabSymbolPresent API utility for issue Add is-vertical-tab-symbol-present API utility #4833
• use the issue's kebab-case naming convention: is-vertical-tab-symbol-present.ts
• add a focused Node test file covering positive, negative, and literal-escape cases
• wire the API workspace npm test script to execute the new test file
• register the AI-assisted contribution as required by the repository guidelines

Validation

• npm test — API utility tests: 3 passed; other workspaces retain their existing placeholder test commands
• additional runtime validation — 11 deterministic cases and 10,000 randomized cases passed
• strict TypeScript compilation passed for the implementation and test file
• contributors/agents.json JSON validation passed with PR [agent] feat: add vertical tab symbol utility #9195 and issue Add is-vertical-tab-symbol-present API utility #4833
• git diff --check passed

Closes #4833

[FurinaDog]

Maintainer review request: this PR is focused on #4833, follows the repository AI-agent requirements, and is currently CLEAN. Validation completed: npm test (3/3 passed), runtime cases, and git diff --check. Please let me know if any changes are needed for merge and bounty processing.