feat: expose cache-primary-key output (#597) [v5 backport] (#1089)

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

Backport of the cache-primary-key output to the v5 release line.

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 (CommonJS)

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

* Apply PR #1088 review suggestions to v5 backport

Port the review feedback from the main-line PR (#1088) into the v5 backport:
- src/cache.ts: use the STATE_CACHE_PRIMARY_KEY constant for the output name
  instead of a duplicated string literal
- action.yml / README.md: clarify that the output is also empty when caching
  is skipped (e.g. the cache service is unavailable)
- dist: rebuild

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

---------

Co-authored-by: Copilot App <223556219+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'