Refine payload base and add backup agent job builders

[svdigital-development]

Summary

• type the payload contract and turn GenericPayload into an abstract base with JSON helpers and documentation updates
• add AbstractJsonPayload, JsonPatchPayload, and dedicated Windows/Linux backup agent job payload builders
• wire BackupAgentJobRepository to the new payload classes and document the changes in the changelog and README

Testing

• php -l src/Payloads/GenericPayload.php
• php -l src/Payloads/AbstractJsonPayload.php
• php -l src/Payloads/JsonPatchPayload.php
• php -l src/Payloads/CreateWindowsBackupAgentJobConfigurationPayload.php
• php -l src/Payloads/CreateLinuxBackupAgentJobConfigurationPayload.php
• php -l src/Payloads/EditWindowsBackupAgentJobConfigurationPayload.php
• php -l src/Payloads/EditWindowsBackupAgentJobPayload.php
• php -l src/Payloads/EditLinuxBackupAgentJobConfigurationPayload.php

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https://chatgpt.com/codex/tasks/task_b_68ed19754fd4832ea2a389440938ecb9