Fix DST transition issue causing forced caching of non-cacheable responses

[Copilot]

This PR fixes a critical issue where responses that should not be cached were being cached for approximately one hour during DST (Daylight Saving Time) transitions when PHP is configured with a non-UTC timezone.

Problem

The library uses new \DateTime('-1 seconds') to create expiry timestamps for cache entries that should not be cached (e.g., responses with Cache-Control: no-cache). During DST transitions, specifically when going from summer time to winter time, there's an hour that repeats. PHP's DateTime class with relative time strings can create ambiguous timestamps that resolve to future times instead of past times.

For example, during the Europe/Berlin DST transition on 2024-10-27:

• Both UTC times "2024-10-27 00:00:56" and "2024-10-27 01:00:56" correspond to "2024-10-27 02:00:56" in local time
• new \DateTime('-1 seconds') could resolve to the later occurrence, creating a future timestamp
• This results in a TTL of ~3599 seconds instead of -1, causing unwanted caching

Solution

Replace new \DateTime('-1 seconds') with new \DateTime('@' . (time() - 1)) in two locations within PrivateCacheStrategy.php. The '@' prefix creates a DateTime object from a Unix timestamp, which is always UTC-based and unambiguous.

Changes

• src/Strategy/PrivateCacheStrategy.php: Fixed two instances where relative DateTime creation was replaced with explicit UTC timestamps
• tests/DstTransitionTest.php: Added comprehensive tests to verify the fix works across different timezones and prevents the DST issue

Impact

• ✅ Cache entries marked for immediate expiry now consistently expire immediately
• ✅ No unexpected caching during DST transitions
• ✅ Consistent behavior across all timezones
• ✅ Maintains all existing functionality - this is a surgical fix

The fix ensures RFC 7234 compliance is maintained while preventing the timezone-related edge case that could cause security or correctness issues.

Fixes #194.

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[Copilot]

Pull Request Overview

This PR fixes a critical DST (Daylight Saving Time) bug where responses marked as non-cacheable were incorrectly cached for approximately one hour during timezone transitions. The issue occurred because new \DateTime('-1 seconds') could resolve to future timestamps during DST transitions when PHP uses non-UTC timezones.

• Replaced relative DateTime creation with UTC timestamp-based approach in PrivateCacheStrategy
• Added comprehensive test coverage to verify the fix across different timezones
• Ensures RFC 7234 compliance is maintained while preventing timezone-related edge cases

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Strategy/PrivateCacheStrategy.php	Fixed two instances of relative DateTime creation that could cause DST issues by using UTC timestamps
tests/DstTransitionTest.php	Added new test class to verify the fix works across timezones and prevents DST-related caching issues