Improve cache service availability determination and change failure log levels

[Copilot]

This PR improves how the cache service availability is determined in @actions/toolkit and changes the log level of cache failures from warnings to errors.

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 either ACTIONS_CACHE_URL or ACTIONS_RESULTS_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'] || process.env['ACTIONS_RESULTS_URL']
      )
  }
}

2. Changed log levels from warnings to errors

Updated all cache failure logs from core.warning() to core.error() in:

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

3. Comprehensive test coverage

Added extensive test coverage including:

• V1 service with different URL combinations
• V2 service with correct and incorrect URL configurations
• GHES behavior with v2 flag enabled
• Edge cases with no URLs set
• Updated existing tests to expect errors instead of warnings

Exit Criteria Met

✅ Even if ACTIONS_CACHE_URL is not set, isFeatureAvailable() now uses alternative ways to determine cache service availability
✅ Changed log level of cache failures from warning to error

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

Fixes #2099.

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[Link-]

Fix the implementation as per the comments provided

[Copilot]

Fix the implementation as per the comments provided

Implemented as requested in commit d48d6b6. Removed the logCacheError function and added inline 5xx server error detection in all four cache functions.