This PR improves how the cache service availability is determined in @actions/toolkit and implements conditional error logging for cache failures based on HTTP status codes.

Changes Made

1. Enhanced isFeatureAvailable() function

The isFeatureAvailable() function now properly leverages the ACTIONS_CACHE_SERVICE_V2 feature flag to determine cache service availability:

• For cache service v2: Requires ACTIONS_RESULTS_URL to be set
• For cache service v1: Requires ACTIONS_CACHE_URL to be set
• For GHES: Always uses v1 logic regardless of the v2 flag setting

Before:

export function isFeatureAvailable(): boolean {
  return !!process.env['ACTIONS_CACHE_URL']
}

After:

export function isFeatureAvailable(): boolean {
  const cacheServiceVersion = getCacheServiceVersion()
  
  switch (cacheServiceVersion) {
    case 'v2':
      return !!process.env['ACTIONS_RESULTS_URL']
    case 'v1':
    default:
      return !!process.env['ACTIONS_CACHE_URL']
  }
}

2. Conditional error logging for cache failures

Updated cache failure logging to differentiate between server errors and other failures:

• Server errors (5xx status codes): Logged as errors using core.error()
• All other cache failures: Logged as warnings using core.warning()

Applied to all cache functions:

• restoreCacheV1() function
• restoreCacheV2() function
• saveCacheV1() function
• saveCacheV2() function

Implementation:

// Log server errors (5xx) as errors, all other errors as warnings
if (
  typedError instanceof HttpClientError &&
  typeof typedError.statusCode === 'number' &&
  typedError.statusCode >= 500
) {
  core.error(`Failed to restore: ${(error as Error).message}`)
} else {
  core.warning(`Failed to restore: ${(error as Error).message}`)
}

3. Comprehensive test coverage

Added extensive test coverage including:

• V1 service with ACTIONS_CACHE_URL requirements
• V2 service with ACTIONS_RESULTS_URL requirements
• GHES behavior with v2 flag enabled
• Server error (5xx) vs other error logging differentiation
• Edge cases with no URLs set
• Updated existing tests to expect appropriate log levels

Backward Compatibility

This change maintains full backward compatibility:

• Existing workflows using ACTIONS_CACHE_URL continue to work unchanged
• V1 cache service behavior is preserved
• GHES installations are unaffected
• Only server errors (5xx) are elevated to error level, preserving existing warning behavior for most cache failures

Fixes #2099.

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