feat: expose cache-primary-key output (#597) (#1088)

* feat: expose cache-primary-key output (#597)

Expose the primary cache key computed by the caching logic as a new
`cache-primary-key` action output, so workflows can compose the built-in
setup-java cache key with actions/cache or actions/cache/restore across
steps and dependent jobs.

- src/cache.ts: set the `cache-primary-key` output in restore()
- action.yml: declare the new output
- README.md: document the new output
- __tests__/cache.test.ts: assert the output is set
- dist: rebuild

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* chore: rebuild dist

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

action.yml

@@ -103,6 +103,8 @@ outputs:
     description: 'Path to where the java environment has been installed (same as $JAVA_HOME)'
   cache-hit:
     description: 'A boolean value to indicate an exact match was found for the primary key'
+  cache-primary-key:
+    description: 'The primary cache key computed by the action for the configured build tool. Empty when caching is not enabled or when caching is skipped (e.g. cache service unavailable). Useful for composing with actions/cache or actions/cache/restore across jobs.'
 runs:
   using: 'node24'
   main: 'dist/setup/index.js'