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>

__tests__/cache.test.ts

@@ -78,6 +78,10 @@ describe('dependency cache', () => {
       ReturnType<typeof glob.hashFiles>,
       Parameters<typeof glob.hashFiles>
     >;
+    let spySetOutput: jest.SpyInstance<
+      ReturnType<typeof core.setOutput>,
+      Parameters<typeof core.setOutput>
+    >;
 
     beforeEach(() => {
       spyCacheRestore = jest
@@ -86,6 +90,7 @@ describe('dependency cache', () => {
           Promise.resolve(undefined)
         );
       spyGlobHashFiles = jest.spyOn(glob, 'hashFiles');
+      spySetOutput = jest.spyOn(core, 'setOutput').mockImplementation(() => {});
       spyWarning.mockImplementation(() => null);
     });
 
@@ -120,6 +125,15 @@ describe('dependency cache', () => {
         expect(spyWarning).not.toHaveBeenCalled();
         expect(spyInfo).toHaveBeenCalledWith('maven cache is not found');
       });
+      it('sets the cache-primary-key output', async () => {
+        createFile(join(workspace, 'pom.xml'));
+
+        await restore('maven', '');
+        expect(spySetOutput).toHaveBeenCalledWith(
+          'cache-primary-key',
+          expect.stringContaining('setup-java-')
+        );
+      });
       it('downloads cache based on maven-wrapper.properties', async () => {
         createDirectory(join(workspace, '.mvn'));
         createDirectory(join(workspace, '.mvn', 'wrapper'));