feat: backport cache read-denied + ACTIONS_CACHE_MODE handling (5.2.0)

Backport of the read-denied and ACTIONS_CACHE_MODE cache-mode gating from
the ESM v6.2.0 line (#2447) to the CommonJS v5 line, released as 5.2.0.
Mirrors the earlier write-denied backport (#2435, 5.1.0).

- Detect the `cache read denied:` prefix on download failures (v2 twirp
  path and v1 `_apis/artifactcache` path) and surface it as a core.warning
  without failing the run.
- Honor ACTIONS_CACHE_MODE: skip restore when the effective cache-mode does
  not permit reads (none, write-only) and skip save when it does not permit
  writes (none, read), logging a single non-fatal core.info line. Unset or
  unrecognized modes are unchanged.
- Add read-denied and cache-mode tests; bump to 5.2.0 with RELEASES entry.

Co-authored-by: Copilot App <[email protected]>
Copilot-Session: e96deec1-716e-4e14-acdf-a230139420a2

packages/cache/__tests__/config.test.ts

@@ -23,3 +23,37 @@ test('isGhes returns false for ghe.localhost', () => {
   process.env.GITHUB_SERVER_URL = 'https://my.domain.ghe.localhost'
   expect(config.isGhes()).toBe(false)
 })
+
+describe('cache-mode helpers', () => {
+  const original = process.env.ACTIONS_CACHE_MODE
+
+  afterEach(() => {
+    if (original === undefined) {
+      delete process.env.ACTIONS_CACHE_MODE
+    } else {
+      process.env.ACTIONS_CACHE_MODE = original
+    }
+  })
+
+  test('getCacheMode normalizes whitespace and case', () => {
+    process.env.ACTIONS_CACHE_MODE = '  Write-Only  '
+    expect(config.getCacheMode()).toBe('write-only')
+  })
+
+  test('getCacheMode returns empty string when unset', () => {
+    delete process.env.ACTIONS_CACHE_MODE
+    expect(config.getCacheMode()).toBe('')
+  })
+
+  test.each([
+    ['', true, true],
+    ['read', true, false],
+    ['write', true, true],
+    ['write-only', false, true],
+    ['none', false, false],
+    ['garbage', true, true]
+  ])("mode '%s' -> readable=%s writable=%s", (mode, readable, writable) => {
+    expect(config.isCacheReadable(mode)).toBe(readable)
+    expect(config.isCacheWritable(mode)).toBe(writable)
+  })
+})