Improve test coverage with comprehensive edge case testing

[tomaj]

Summary

• Significantly improved test coverage from 40 to 55 tests (+37.5% increase)
• Modernized test suite to use PHP 8 attributes (#[Test], #[CoversClass])
• Added comprehensive edge case testing for all parsers and MailContent class
• Enhanced error handling validation for invalid inputs and boundary conditions
• Created dedicated MailContentTest class for core data structure testing

New Test Coverage Areas

MailContent Edge Case Testing

• Empty string handling in setters (setKs, setSs, setVs, setCc, setTid)
• Null state verification for all getters on fresh instances
• Complete property coverage testing all setters and getters
• Edge case validation ensuring empty strings convert to null appropriately

VUB Parser Robustness

• Invalid email content handling (non-VUB format emails)
• Empty content behavior verification
• Partial field parsing when only some fields are present
• Invalid amount format handling (floatval edge cases)

TatraBanka Simple Parser Edge Cases

• Empty string values in key=value pairs
• Empty content null return verification
• Unknown parameters filtering (tests parameter map lookup logic)

ČSOB Parser Error Conditions

• No transaction marker detection (empty result when regex fails)
• Invalid date format graceful handling
• Non-matching email format rejection
• Empty content proper handling

Technical Improvements

Coverage Statistics

• Tests: 40 → 55 (+15 tests, +37.5%)
• Assertions: 338 → 405 (+67 assertions, +19.9%)
• New test categories: 4 major areas of edge case coverage
• Error path testing: All parsers now test failure scenarios

Modernization

• PHP 8 attributes: Replaced @test annotations with #[Test] attributes
• Class coverage: Added #[CoversClass] attributes for better organization
• Method naming: Removed test prefixes from method names
• Clean code: Removed redundant comments

Test plan

• All 55 tests pass
• PSR-2 code style compliance verified
• No breaking changes to existing functionality