Add read-only mode for dependency caching

[brunoborges]

Cache-enabled pull requests, merge queues, short-lived branches, and fan-out matrix jobs can waste post-job time and storage by uploading duplicate or temporary cache archives. This adds a restore-only mode so those jobs can consume reusable caches while a default-branch or seed job remains responsible for writes.

Changes

• Add a cache-read-only input that defaults to false.
• Preserve normal Maven, Gradle, sbt, and wrapper-cache restores while gating the post action before any cache save occurs.
• Log clearly when cache persistence is skipped because read-only mode is enabled.
• Cover all supported package managers in unit and end-to-end tests.
• Document default-branch writer and seed-job/read-only matrix patterns.
• Regenerate the checked-in action bundles.

Validation

• npm run check
• 1,191 unit tests passing

Fixes: #1168

[Copilot]

Pull request overview

Adds a first-class “restore-only” caching mode to actions/setup-java so cache restores still happen, but cache saves (including wrapper caches) are skipped in the post step. This reduces duplicate cache uploads in PRs, merge queues, and fan-out matrix jobs while keeping a default-branch/seed job responsible for writing caches.

Changes:

• Introduces a new cache-read-only input (default false) and documents recommended workflow patterns.
• Gates the post-job cache save in cleanup-java when cache-read-only is enabled, with an explicit log message.
• Updates unit tests and the e2e cache workflow to exercise read-only restores; regenerates dist/ bundles.

Show a summary per file

File	Description
src/constants.ts	Adds the cache-read-only input constant.
src/cleanup-java.ts	Skips cache saving in the post action when cache-read-only is true.
README.md	Documents cache-read-only and provides default-branch writer + seed/matrix patterns.
action.yml	Declares the new cache-read-only input with default false.
tests/cleanup-java.test.ts	Adds unit coverage ensuring cache saves are skipped in read-only mode and still occur when explicitly disabled.
.github/workflows/e2e-cache.yml	Runs restore jobs in read-only mode to validate restores without post-job saves.
dist/setup/index.js	Updates bundled constant for cache-read-only.
dist/cleanup/index.js	Updates bundled cleanup logic to honor cache-read-only.

Review details

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• Files reviewed: 6/8 changed files
• Comments generated: 0
• Review effort level: Low