test: Add unit tests for stringHelper.contains and randomAlphanumeric

[olitreadwell]

What changed

Adds unit tests for two uncovered helpers in common/components/utils/string.js, both in common/components/test/string.test.js:

• contains: one test, four assertions (true case, false case, empty-list edge case).
• randomAlphanumeric: one test, three assertions (returns a string, returns a non-empty string, returns a different value across calls).

No source code changed.

Why

common/components/utils/string.js exports six static helpers.

The existing test file covered four of them (isEmptyOrWhitespace, trimStartString, startsWithAny, isValidSlug) but not contains or randomAlphanumeric.

The source file also carries a // TODO: Update unit tests comment.

So both helpers had zero coverage.

These tests lock in their behaviour.

How to verify

git fetch https://github.com/olitreadwell/CivicTechExchange.git test/civictech/add_stringhelper_contains_test
git checkout FETCH_HEAD
npm ci --ignore-scripts
npx jest common/components/test/string.test.js

Expected: 6 passed (was 4 before this change).

Coverage

N/A: the repo has no coverage tool wired up.

Method-level: contains and randomAlphanumeric go from 0 tests to 1 each.

Checks run locally

• Tests: PASS (full suite: 25 passed, 3 skipped, 0 failed; was 23 passed before)
• Lint / format: PASS (npx prettier --check clean on the changed file)
• Typecheck: N/A (repo uses Flow; no typecheck script, and no types were changed)
• Build: N/A (webpack build needs a Python/Django backend + collectstatic; a test-only change does not touch build output)