Upgrade slevomat/coding-standard to 8.27.* and add ClassKeywordOrder sniff

[Copilot]

Upgrades slevomat/coding-standard from 8.25.* to 8.27.* to support the latest 8.27.1 release, and adds the new ClassKeywordOrder sniff to the ruleset.

Changes

• Updated composer.json dependency constraint to 8.27.*
• Updated CHANGELOG.md with version bump and new rule documentation
• Added SlevomatCodingStandard.Classes.ClassKeywordOrder rule to ruleset.xml
• Created test fixtures for ClassKeywordOrder sniff (compliant and violation cases)
• Added 2 new unit tests to verify the ClassKeywordOrder sniff functionality

Notes

• Non-breaking upgrade: all tests pass without modification
• Key improvements in 8.27.x: #[Override] attribute handling, new ClassKeywordOrder sniff, bug fixes for DocCommentHelper
• The ClassKeywordOrder sniff enforces correct order of class modifiers: (final | abstract) readonly class
• All 18 tests pass (increased from 16 tests)
• The sniff is fixable and can automatically correct modifier order violations

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