fix: honor omitted primaries-required input

[dale-lakes]

Summary

• Treat an omitted primaries-required input as disabled.
• Keep online busy primary runners eligible when that option is omitted, allowing jobs to queue for them.
• Add regression coverage and rebuild the checked-in action bundle.

Root cause

@actions/core.getInput() returns an empty string for an omitted optional input. The availability check only distinguished undefined, so the empty string entered the primaries-required branch and skipped every busy runner.

Any non-empty primaries-required value retains the existing free-runner count behavior.

Validation

• npm exec --yes --package=npm@10 -- npm run all
• ESLint passed.
• Action bundle rebuilt with ncc.
• Jest: 6 tests passed.