Fix/agent patch config defaults

[ozgen]

What

• Default missing scan agent config fields to the agent’s existing configuration when building the patch payload.
• Treat 0 values in update->config as “not set” and inherit from the agent config.
• Deep-copy scheduler_cron_time to avoid shared ownership.
• Added unit tests covering merge logic, deep-copy behavior, and guard conditions.

Why

Previously, when update->config was partially provided, missing fields were not defaulted from the agent’s current configuration, which could lead to unintended zero values being sent in the patch payload.

References

GEA-1625

Checklist

• Tests

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

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