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

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* 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

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* Refine JDK cache semantics

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* 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.

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* Align JDK cache OS identity

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

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* 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

action.yml

@@ -84,6 +84,9 @@ inputs:
   cache:
     description: 'Name of the build platform to cache dependencies. It can be "maven", "gradle" or "sbt".'
     required: false
+  cache-jdk:
+    description: 'Cache downloaded JDK installations between jobs. Defaults to enabled when dependency caching is configured with `cache`; set explicitly to "true" or "false" to override.'
+    required: false
   cache-dependency-path:
     description: 'The path to a dependency file: pom.xml, build.gradle, build.sbt, etc. This option can be used with the `cache` option. If this option is omitted, the action searches for the dependency file in the entire repository. This option supports wildcards and a list of file names for caching multiple dependencies.'
     required: false
@@ -91,7 +94,7 @@ inputs:
     description: 'The path to cache instead of the default dependency cache path for the selected package manager. This option can be used with the `cache` option and supports a list of paths and exclusion patterns.'
     required: false
   cache-read-only:
-    description: 'Restore dependency caches without saving cache changes in the post action.'
+    description: 'Restore caches without saving cache changes in the post action.'
     required: false
     default: false
   job-status: