Add conditional JDK caching (#1201)

* Add JDK caching

Cache resolved JDK tool-cache entries by exact platform and release identity, with a default-on cache-jdk input and explicit opt-out.

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

* Apply batched suggestions from code review

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

* Fix JDK cache CI validation

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

* Update brace-expansion security fix

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

* Refresh brace-expansion license metadata

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

* Refine JDK cache semantics

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

* Refine JDK cache documentation

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

* Simplify JDK cache identity

Use one normalized runner OS dimension, reset the internal cache key schema for the unreleased feature, and align documentation, tests, and bundles.

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

* Align JDK cache OS identity

Use the established RUNNER_OS value directly and retain process.platform only as a non-Actions fallback.

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

* Harden JDK cache saves and document tool-cache reuse

Bind each JDK cache key to the installation identity it was computed for,
keep post-job saves best-effort per entry, and state the real reuse and
verification guarantee in the documentation.

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

* docs: restructure README caching section

Rename '## Caching dependencies' to '## Caching' and add a what-gets-cached
overview table covering the dependency, wrapper, and JDK caches. Lead with the
common 'cache: maven' example and the dependency-cache material, and demote JDK
caching into its own subsection.

Also corrects the IMPORTANT callout, which implied JDK caching required an
explicit opt-in; it is enabled implicitly whenever 'cache' is set.

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

* docs: fix caching documentation defects

- Remove pull-request framing that compared behavior to `main`; state the
  tool-cache and `jdkfile` behavior directly and unconditionally.
- Clarify that the JDK cache is a separate cache *entry* from the dependency
  and wrapper caches, while its *enablement* is coupled to `cache`, so the
  opening paragraph agrees with the enablement matrix.
- Cite the actions/setup-java-benchmarks repository instead of an open PR and
  a self-referential PR comment, keeping the measured figures and caveats.
- Keep the `cache`/`cache-jdk` matrix only in docs/advanced-usage.md and
  summarize the rules in prose in README.md to avoid divergence.
- Describe the guarantee that a cache key is only saved with the installation
  it was computed for, instead of documenting inode/size/timestamp internals.

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

* docs: add V6 what's new entry for JDK caching

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: e2755464-4e83-47b6-ba71-731bb481b418

---------

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>
Copilot-Session: e2755464-4e83-47b6-ba71-731bb481b418

__tests__/util.test.ts

@@ -49,7 +49,8 @@ const {
   isGhes,
   validatePaginationUrl,
   getLatestMajorVersion,
-  getBooleanInput
+  getBooleanInput,
+  isJdkCacheEnabled
 } = await import('../src/util.js');
 
 describe('getBooleanInput', () => {
@@ -115,6 +116,37 @@ describe('getBooleanInput', () => {
   });
 });
 
+describe('isJdkCacheEnabled', () => {
+  let inputs: Record<string, string>;
+
+  beforeEach(() => {
+    inputs = {};
+    (core.getInput as jest.Mock).mockImplementation(
+      (name: string) => inputs[name] ?? ''
+    );
+  });
+
+  afterEach(() => {
+    jest.resetAllMocks();
+  });
+
+  it.each([
+    ['', '', false],
+    ['', 'true', true],
+    ['', 'false', false],
+    ['maven', '', true],
+    ['maven', 'true', true],
+    ['maven', 'false', false]
+  ])(
+    'resolves cache=%j and cache-jdk=%j to %s',
+    (cache, cacheJdk, expected) => {
+      inputs['cache-jdk'] = cacheJdk;
+
+      expect(isJdkCacheEnabled(cache)).toBe(expected);
+    }
+  );
+});
+
 describe('isVersionSatisfies', () => {
   it.each([
     ['x', '11.0.0', true],