chore(maintenance): scope test:unit to tests/unit and align cached-node-modules action pins

[Larslllllll]

Summary

Two maintenance fixes:

Fix 1: Scope test:unit to tests/unit (#5643)

Six packages (commons, batch, jmespath, kafka, testing, validation) defined
test:unit as bare vitest --run. Without an explicit include path,
the default glob matches tests/e2e/*.test.ts as well, so npm run test:unit
in the batch package runs the SQS e2e suite which requires AWS credentials.

Fix: Update test:unit to vitest --run tests/unit in all six affected
packages. Also normalises test:e2e and test:unit:types placeholder strings.

Fix 2: Align cached-node-modules action pins (#5642)

The cached-node-modules composite action is pinned at two different commits
across workflows: the older January 2025 pin (29979bc5) has no version
comment, so Dependabot never proposes bumping it. The newer August 2025 pin
(3b5b8e2e, v1.5.0) carries the comment.

Fix: Update all 13 uses of the old pin to v1.5.0 with a # v1.5.0
comment, so all workflows use the same version and Dependabot can track them.

Fixes #5643, #5642

[powertools-for-aws-oss-automation]

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[svozza]

Coudl you use the create-pr skill so that we follow the repo conventions? Also, one fix per PR please.

[dreamorosi]

Thank you for taking the time to contribute! We've decided to handle this one as maintainers since it touches conventions we're standardizing across the repo, and it's now addressed in #5653. The GitHub Actions pin changes bundled here are out of scope for #5643 — if there's a concrete problem behind them, please open a separate issue so we can look at it on its own merits. Closing this in favor of that PR — we'd be happy to see you pick up another issue.