Refactor: extract shared utils, fix logging, async pattern, and missing guards

[Copilot]

fetch.js and save.js duplicated boolean parsing and cache key/path building logic, used console.log instead of core logging APIs, mixed async/await with .then()/.catch(), and had several correctness gaps (redundant process.exit(1), no guard against empty paths, save.js ignoring the clean input entirely).

Changes

New utils.js

• parseBooleanInput — consolidated from both files
• buildBaseConfig — consolidates prefix chdir, key string construction, and toolchain path building shared by both steps

fetch.js

• Uses utils.js; removes local duplication
• console.log(keyString, restoreKeys) → core.debug(...) with individual labeled entries
• Removed redundant process.exit(1) after core.setFailed() (already sets exit code 1)
• Skips cache.restoreCache when paths is empty

save.js

• Uses utils.js; removes local duplication
• Adds early return when clean=true — previously the input was silently ignored, risking an unintended save after a cache-busting run
• Splits the combined skip condition into three explicit early returns, each with a core.debug message
• Replaces .then()/.catch() chained onto await with pure async/await
• console.log → core.info / core.debug
• Skips cache.saveCache when paths is empty

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