test: improve mutation coverage for certificate validation edge cases

[UmairRathore]

Related to #8053

📝 Summary

Improves mutation-test coverage for ValidateService by adding focused regression cases around certificate field validation boundaries.

The added tests cover:

• whitespace-only values for required string fields
• whitespace-only entries in optional array fields
• the exact OU maximum item boundary of 10
• invalid non-string values inside OU

No production code is changed.

Mutation testing results

Metric	Before	After
PHPUnit tests	27	31
PHPUnit assertions	41	47
Generated mutants	78	80
Killed mutants	66	72
Escaped mutants	12	8
Mutation Code Coverage	100%	100%
Covered Code MSI	84%	90%

The generated-mutant count changed between runs as the additional tests exercised more behavior, so the most relevant comparison is the reduction from 12 to 8 escaped mutants and the Covered Code MSI improvement from 84% to 90%.

The remaining escaped mutants correspond to behavior that is currently non-observable with the existing certificate rules, primarily because OU is optional (required = false) and has minItems = 0. I did not add artificial test cases or change mutation configuration solely to increase the score.

🧪 How to test

Run the focused PHPUnit tests:

composer test:unit -- --filter ValidateServiceTest

Expected result:

OK (31 tests, 47 assertions)

Run Infection against the corresponding source file:

composer mutation:test -- \
  lib/Service/Certificate/ValidateService.php \
  --test-framework-options="--filter ValidateServiceTest" \
  --show-mutations \
  --no-progress \
  --no-interaction

Expected mutation-testing summary:

Mutation Code Coverage: 100%
Covered Code MSI: 90%

The focused run generates 80 mutants, with 72 killed and 8 escaped.

⚙️ API / Back-end changes

• Added certificate validation boundary and invalid-input test coverage
• Unit tests added
• Capabilities updated (not applicable)
• Documentation updated (not applicable)
• API documentation updated (not applicable)

✅ Checklist

• I have read and followed the contribution guide.
• Focused PHPUnit tests pass.
• Focused Infection run completed successfully.
• No production behavior was changed.
• No mutation exclusions, disabled mutators, or threshold changes were added.
• Commit includes DCO sign-off.

🤖 AI (if applicable)

• The content of this PR was partially or fully generated using AI

[vitormattos]

Very nice work! Thanks!

[vitormattos]

/backport to stable34

[vitormattos]

/backport to stable33