Fix certificate assignment for non-default services

[jmcte]

🗃️ Summary

Fix certificate assignment for DSM services that are discovered from the live certificate service list instead of the static service map.

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🚀 Motivation & Problem Statement

Certificate.set_certificate_for_service() currently indexes a hardcoded service dictionary by service_name. That works for DSM Desktop Service, but it fails for non-default DSM services such as reverse proxy certificate assignments because those service entries are returned by DSM at runtime and are not present in the static map.

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🔧 Implementation Details

• Modified synology_api/core_certificate.py so set_certificate_for_service() captures the matching service object while scanning list_cert().
• Kept the existing static payload behavior for known services such as DSM Desktop Service.
• Added a fallback that uses the DSM-discovered service definition for non-default services.
• Added a clear ValueError when the requested service is not present in the certificate service list.
• Added focused unit tests in tests/test_core_certificate.py for default service behavior, discovered non-default service behavior, missing service errors, and already-assigned no-op behavior.

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🏁 Checklist

• I have read and followed the Contributing guidelines.
• All new or modified code is covered by unit tests (tests/).
• Tests pass locally (pytest).
• I added or updated documentation where necessary.
• I updated the changelog or added a new section if this is a major change.
• I followed the style guidelines (black, flake8, etc.).
• I ran pre-commit and addressed any linting issues.

pre-commit is not installed in the project venv used for local verification, so I could not run it locally. The GitHub pre-commit workflow is running on this PR.

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📝 Related Issue

Closes #341.

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🔨 Additional Notes

Live DSM verification was performed against reverse proxy certificate assignment:

• Created a temporary reverse proxy service.
• Changed its certificate assignment.
• Verified the new assignment was reflected by DSM.
• Restored the original certificate assignment.
• Deleted the temporary reverse proxy service.

The marked safe integration tests currently fail for me even on clean upstream/master against my NAS, so I am treating those as existing environment/API-version drift rather than part of this change.

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😎 Test & Build Status

python -m pytest tests/test_core_certificate.py tests/test_core_service_apps.py -q
# 64 passed in 0.09s

git diff --check
# passed

Attempted but unavailable locally:

pre-commit run --all-files
# no such file or directory

python -m pre_commit run --all-files
# No module named pre_commit

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👀 Screenshots / Media

Not applicable.

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Thank you for contributing!